The relationship of actinically-induced "premature aging" to chronological aging was studied in paired keratinocyte cultures obtained from the habitually sun-exposed (lateral) and nonexposed (medial) aspects of the arm of 5 male donors, aged 41 to 80 yr. In all cases, the number of cell generations in vitro was greater for cultures derived from sun-exposed skin, and this discrepancy increased with donor age and the severity of clinical aging changes. Hence, chronic sun exposure does accelerate aging in human skin by at least one previously established in vitro criterion: it decreased the lifespan of cultured keratinocytes. Plating efficiency was 11- to 32-fold higher for keratinocytes from chronically sun-exposed skin than nonexposed controls, perhaps reflecting the recognized carcinogenic potential of actinic radiation. Keratinocyte cultures appear to be as amenable to gerontologic studies as the already widely used human fibroblast cultures.