OBJECTIVE To investigate changes in incidence of in-situ, 'thin' (< 0.76 mm) and 'thick' (> 0.76 mm) melanoma. To explore the relationship of melanoma depth with the patient's age at diagnosis. METHODS A retrospective case series of melanoma was taken from pathology records in Palmerston North between 1983 and 1994. The denominator population was estimated, for each year, from published 1986 and 1991 census figures. RESULTS Over the 12 years, the diagnostic rate of in-situ and invasive melanomas increased annually by 16% and 5%, respectively (Poisson regression, p < 0.0001). This analysis did not show a difference between the overall rate of increase in 'thin' and 'thick' invasive melanomas. After excluding Hutchinson's melanoma, the mean age at diagnosis for 'thin' and 'thick' melanomas was 49.0 and 55.4 years, respectively (p < 0.0001); the peak number of 'thin' and 'thick' melanomas were in the 30-39 and 60-69 age groups, respectively. CONCLUSIONS There is evidence for progression from 'thin' to 'thick' melanoma over a 6-to 30-year period. Poor prognosis 'thick' melanomas are currently increasing. Providing removal of 'thin' melanomas is sustained, a decline in 'thick' melanomas is expected but may take decades.