Metastasis by thin melanomas is uncommon and unpredictable. In order to assess the prognostic value of p53 expression, p53 immunohistochemical staining was evaluated in 20 thin melanomas with documented metastasis and in 20 control tumours which failed to metastasize. Tumours selected were less than 1 mm thick and were individually matched for tumour thickness, date of excision and patient age and sex. The relative risk of metastasis given p53 overexpression was 1.5 (95% confidence interval 0.4-5.3; p = 0.53). Further quantitative analysis for p53 between the two groups did not demonstrate a significant difference (p = 0.08). These results are consistent with there being no association between p53 overexpression in thin melanomas and risk of metastasis, however, the sample size was small, and the existence of such an association cannot be ruled out with confidence. For the 20 thin melanomas which metastasized, there was no association between the proportion of cells positive for p53 and length of the relapse-free period (correlation coefficient = 0.02, p = 0.94).