The Altcoin Season Index at 17–18: Is It Time to Double Down on Bitcoin or Prepare for Altcoin Rotation?

The Altcoin Season Index (ASI) currently hovers between 17 and 18, a level historically associated with "Bitcoin Season", where capital flows predominantly into Bitcoin while altcoins lag behind. This metric, calculated by comparing the 90-day performance of the top 100 altcoins (excluding stablecoins) to Bitcoin, serves as a critical barometer for market sentiment and capital allocation. With Bitcoin's dominance (BTC.D) rising to 57% in late 2025 from 65% earlier in the year, investors face a pivotal question: Should they double down on Bitcoin's strength or position for an eventual altcoin rotation?

Understanding the Altcoin Season Index and Market Cycles

The ASI operates on a binary framework: a score above 75% signals an "Altcoin Season," characterized by widespread altcoin outperformance, while scores below 25% indicate a "Bitcoin Season," where Bitcoin consolidates its dominance and altcoin activity wanes. Historically, Bitcoin Seasons often follow Bitcoin-driven bull runs or periods of macroeconomic uncertainty, as investors seek the safety of the largest cryptocurrency. For example, during the 2017 and 2021 Altcoin Seasons, Bitcoin's dominance fell to 32% and 37%, respectively, while altcoins surged in tandem with broader market optimism. Conversely, when the ASI drops to levels like 17–18, as it has in late 2025, Bitcoin's dominance typically rises to 60–70%, reflecting a flight to quality.

Historical Context: Lessons from Past Cycles

The 2017 and 2021 Altcoin Seasons offer instructive parallels. In 2017, Bitcoin's dominance plummeted from 85% to 37% as altcoins like Ethereum, Ripple, and Litecoin surged over 1,000%. Similarly, the 2021 Altcoin Season saw DeFi tokens and meme coins outperform Bitcoin by margins of 174% as BTC.D fell to 32% according to data. These cycles were driven by Bitcoin's consolidation phases, which freed up capital to flow into altcoins. However, the current Bitcoin Season, marked by an ASI of 17–18, differs in key ways. Unlike previous cycles, macroeconomic uncertainty and the recent approval of Bitcoin ETFs have amplified Bitcoin's appeal as a store of value, with institutional capital flocking to its perceived stability.

Current Market Analysis: Why the ASI is at 17–18

Several factors explain the ASI's current trajectory. First, Bitcoin's recent stabilization near $90,000 has drawn capital away from riskier altcoins, as investors prioritize liquidity and downside protection. Second, the success of Bitcoin ETFs has created a self-reinforcing cycle: inflows into Bitcoin ETFs boost BTC.D, which in turn suppresses altcoin performance.

Third, macroeconomic headwinds-such as inflation concerns and central bank policies-have heightened risk aversion, favoring Bitcoin's "digital gold" narrative over speculative altcoins.

Strategic Portfolio Positioning: Bitcoin vs. Altcoin Rotation

Given the ASI's current level, strategic portfolio adjustments are warranted. During Bitcoin Seasons, investors are advised to allocate 70–80% of their crypto holdings to Bitcoin, with the remaining 20–30% reserved for high-conviction altcoins with strong fundamentals. For instance, infrastructure tokens (e.g., Solana, Cardano) and projects with robust use cases (e.g., cross-chain bridges, privacy protocols) may outperform even in a Bitcoin-dominated market.

However, preparing for an eventual altcoin rotation requires patience. Historical data suggests that Altcoin Seasons often follow Bitcoin's consolidation phases. For example, in May 2021, the combined market cap of the top 100 altcoins briefly exceeded Bitcoin's by 30%, signaling a shift in capital flows. Investors should monitor BTC.D and the ASI closely, as a rise in altcoin trading volumes and a drop in Bitcoin's dominance below 50% could herald the next Altcoin Season.

Conclusion: Balancing Caution and Opportunity

The ASI at 17–18 underscores a Bitcoin-dominated market, but history shows that such phases are cyclical. While doubling down on Bitcoin remains prudent for risk mitigation, strategic allocations to altcoins with strong fundamentals can position portfolios for the next wave of innovation. As always, disciplined risk management and a long-term perspective are essential. The key is to stay informed, remain agile, and let market cycles guide tactical decisions.