There are many conflicting results concerning the effects of age and Alzheimer's disease (AD) on word-stem completion priming. To examine potential sources of this variability, the authors examined the influences on such priming of age, cognitive status, and encoding in a large sample of young, old, and AD individuals. At study, words were processed aloud by reading, reading and rating likeability, or generating from definition. Old participants had less priming than young participants and more priming than AD patients. For the healthy old participants, priming decreased with advancing age and with cognitive loss following generation only. For AD patients, priming decreased as dementia severity increased; patients with the mildest dementia did not differ from healthy old participants. Thus, age, cognitive status, and encoding differentially influenced the magnitude of priming in healthy aging and AD.