Secular trends in Clark level were investigated by Breslow category for 8,432 invasive cutaneous melanomas diagnosed in South Australia in 1980-2000. More recently diagnosed lesions were found to have deeper levels. After adjusting for age at diagnosis, tumour site, histology, and thickness measured in half millimetres, the relative odds (95% confidence limits) of penetration to the reticular dermis or subcutaneous fat were 1.99 (1.59, 2.50) for the 1987- 93 diagnostic period, and 2.82 (2.25, 3.54) for 1994-2000, when compared with 1980-86. After adjusting for melanoma thickness, the secular trends for deeper lesions applied to a broad cross-section of socio-demographic sub-groups, tumour sites, and histological types. While this similarity in trend would be consistent with a measurement effect, a real change cannot be ruled out and increased emphasis on earlier detection may be warranted. The prognostic implications of changes in inter-relationships between measures of thickness and level require periodic re-evaluation.