Epidermal cell renewal was assessed nonintrusively in normal human volunteers by monitoring the disappearance of a fluorescent marker dye, dansyl chloride, from the skin surface. In young adults, stratum corneum transit time was approximately 20 days, whereas in older adults this was lengthened by more than 10 days. Because the number of horny cell layers does not change with age, these data indicate that the increased stratum corneum transit time was a reflection of diminished epidermal cell proliferation. Additional analysis indicated that the decline in epidermal cell renewal may not occur at a constant rate throughout the adult lifespan but, instead, remains relatively constant in the younger years and then begins to drop dramatically after age 50. This suggests that a linear-spline model rather than a simple linear model may be more appropriate for analyzing these results.