diff --git a/src/p2pool.cpp b/src/p2pool.cpp
index fc1642b..d0ccefd 100644
--- a/src/p2pool.cpp
+++ b/src/p2pool.cpp
@@ -203,6 +203,8 @@ void p2pool::handle_miner_data(MinerData& data)
 		c.reward = 0;
 
 		m_mainchainByHash[c.id] = c;
+
+		cleanup_mainchain_data(data.height);
 	}
 
 	data.tx_backlog.clear();
@@ -1068,6 +1070,31 @@ void p2pool::api_update_stats_mod()
 		});
 }
 
+void p2pool::cleanup_mainchain_data(uint64_t height)
+{
+	// Expects m_mainchainLock to be already locked here
+	// Deletes everything older than 720 blocks, except for the 3 latest RandomX seed heights
+
+	constexpr uint64_t PRUNE_DISTANCE = 720;
+	const uint64_t seed_height = get_seed_height(height);
+	const std::array<uint64_t, 3> seed_heights{ seed_height, seed_height - SEEDHASH_EPOCH_BLOCKS, seed_height - SEEDHASH_EPOCH_BLOCKS * 2 };
+
+	for (auto it = m_mainchainByHeight.begin(); it != m_mainchainByHeight.end();) {
+		const uint64_t h = it->first;
+		if (h + PRUNE_DISTANCE >= height) {
+			break;
+		}
+
+		if (std::find(seed_heights.begin(), seed_heights.end(), h) == seed_heights.end()) {
+			m_mainchainByHash.erase(it->second.id);
+			it = m_mainchainByHeight.erase(it);
+		}
+		else {
+			++it;
+		}
+	}
+}
+
 void p2pool::api_update_block_found(const ChainMain* data)
 {
 	clear_crypto_cache();
diff --git a/src/p2pool.h b/src/p2pool.h
index de44c5a..270ec25 100644
--- a/src/p2pool.h
+++ b/src/p2pool.h
@@ -130,6 +130,8 @@ private:
 	void api_update_pool_stats();
 	void api_update_stats_mod();
 
+	void cleanup_mainchain_data(uint64_t height);
+
 	struct FoundBlock
 	{
 		FORCEINLINE FoundBlock(time_t _t, uint64_t _h, const hash& _id, const difficulty_type& _block_diff, const difficulty_type& _total_hashes)