Young adults, healthy older adults, adults with Parkinson's disease and adults with Alzheimer's disease were given a battery of cognitive tests and a series of verbal fluency tasks including tests of phonetic fluency, semantic fluency and action fluency in both traditional and alternating conditions. Different scoring techniques were compared including counts of correct responses, perseverations, intrusions, and clustering. As expected, young adults produced the most correct responses and the fewest perseverations, while the older adults with Alzheimer's disease produced the fewest correct responses and most perseverations. Cluster size was similar across all groups. The cognitive tests addressed individual differences in processing speed, working memory, inhibition, and verbal ability. Speed and inhibition were the best predictors of performance on verbal fluency measures for the three older groups of adults.