This study characterized the relationship between apolipoprotein E (APOE) status and residual semantic abilities in amnestic mild cognitive impairment (MCI). APOE status (epsilon 4 carrier/non epsilon 4 carrier) was determined in 30 amnestic MCIs and in 22 healthy matched non epsilon 4 carrier controls. The lexical characteristics (age of acquisition, typicality, familiarity) of words produced in a category fluency task were determined. MCIs produced fewer words than controls and these were also earlier acquired and more familiar. The words produced by MCI epsilon 4 carriers were earlier acquired than those of non epsilon 4 carriers. Analyses limited to the first 10 words produced by patients and controls showed similar findings and also revealed that MCI subgroups retrieved first more typical words than controls. Follow up showed higher conversion to Alzheimer's disease (AD) in MCI epsilon 4 carriers than in non epsilon 4 carriers. These findings show that a significant proportion of phenotype variability in performance on category fluency in people at increased AD risk is influenced by genetic factors. These findings explain why category fluency deficits, together with episodic memory deficits, are the only consistent early deficits in MCI patients who convert to AD.