Squamous cell skin cancer, though common, remains largely unreported and unstudied, with little known about its incidence and time trends. We have used a unique resource--a continuous population-based registry of cases of squamous cell skin cancer within a single prepaid health plan-to describe basic epidemiologic features of this malignancy and compare it with the more widely studied melanoma. Both malignancies are considerably more common in this population than we expected based on previous reports from the general population. From the 1960s to the 1980s, the incidence of squamous cell skin cancer increased 2.6 times in men and 3.1 times in women, while incidence of melanoma rose 3.5-fold and 4.6-fold in men and women, respectively. Skin cancers of both types involving the head and neck or the extremities increased essentially in parallel over these 27 years. Melanomas of the trunk, however, appeared to increase at a faster rate in both sexes. These observations are consistent with the impression that the rising incidence of both malignancies may be attributable to increased voluntary exposure to the sun over an extended period.