We conducted a hospital-based case-control study in Brisbane, Queensland, to investigate the specific determinants of basal cell carcinoma (BCC), as distinct from squamous cell carcinoma (SCC), with emphasis on ancestry, residential history, pigmentary characteristics, sun sensitivity and other constitutional factors. The sample was recruited from a dermatology outpatient clinic during an eight-week period in 1991, and comprised 51 incident or recently diagnosed cases of BCC, and 112 randomly selected controls with no known history of BCC or SCC. Twenty-six cases with both BCC and SCC were analysed separately. We found no risk factor specific to BCC which might explain its extremely high prevalence. The strongest risk factors for BCC were advanced age, male sex and a propensity to freckle (independent of skin colour and tendency to burn), all of which have previously been observed for SCC. A finding not previously reported was an apparent protective effect of increasing body mass, specific to BCC alone.