In confirmed late-onset (>65 years) Alzheimer's disease, we found a greater load, both of overall neuritic plaques and of cholinesterase-positive neuritic plaques, in the temporal cortex of carriers of the butyrylcholinesterase K variant (BCHE-K) aged <80 years than of all other patients. The differences were most striking in the case of cholinesterase-positive neuritic plaques. Among BCHE-K carriers, densities of such plaques were over six times higher in patients <80 years at death than in those >80 years (P=0.01). Furthermore, in subjects <80 years, BCHE-K carriers had nearly six-fold greater densities of these plaques than non-carriers (P=0.009). We consider three potential explanations for these findings: that the K variant binds more readily to plaque constituents, that it promotes fibril formation or that it induces aberrant neurite growth.