The DNA content of 14 primary and 111 secondary melanomas was determined by flow cytometry. Aneuploidy was detected in 67% of samples. The frequency with which aneuploid cells were found was similar in primary and metastatic melanomas and, in the metastatic group, for melanotic and amelanotic tumours. Aneuploid diversity was marked with a wide variation in DNA content between tumours. Serial biopsies were performed in 14 patients, and in 10 there was discordance in DNA profiles between first and subsequent biopsies. Tumour biopsies taken from different sites at the same time also showed discordance in 3 of 5 cases. These features highlight the degree of cellular heterogeneity in malignant melanoma.