The objective of this study was to evaluate the time trend of melanoma thickness in a population-based case series. All invasive (n=2862) and in-situ (n=605) cutaneous melanoma incident cases diagnosed in 1985-2004 were retrieved from the Tuscany Cancer Registry, central Italy. Standardized (European population) incidence rates were computed for four periods: 1985-1989, 1990-1994, 1995-1999, 2000-2004, and for Breslow thickness classes (< or =1, 1.01-2.00, >2 mm). The annual percent change (APC) of the standardized rates was computed. Thickness was evaluated on the basis of sex, age, morphology type, site and period of time. Median thickness was evaluated by means of a nonparametric K-sample test. The incidence rate of melanoma rose significantly for both invasive (APC=+5.1%) and in-situ lesions (APC=+11.1). The sex distribution of patients with invasive melanoma did not change over time (mean male/female ratio 0.95). The mean age at diagnosis did not change (57.2 years; SD=17.2 years). From 1985-1989 to 2000-2004 the median value of thickness decreased from 1.68 to 0.8 mm (P<0.001). Within the Breslow categories the median value of thickness decreased significantly for thin melanomas (< or =1 mm) but not for intermediate (1.01-2.00) or for thick melanomas (>2 mm). Among the most common melanoma types, the median thickness decreased for superficial spreading melanomas but not for nodular melanomas. Over time, the incidence of melanoma has increased notably and the median thickness has decreased. However, median thickness has decreased only among thin melanomas, whereas it has not changed for thick melanomas, most of which are of the nodular type.