The DNA stemline of 45 mucinous breast carcinomas was determined by flow cytometry using paraffin embedded archival tissue sections. The material consisted of 26 pure mucinous and 19 mixed mucinous carcinomas. The patients were followed up for at least 15 years or until death. Nearly all pure mucinous carcinomas had a normal DNA stemline (25 of 26) with only one aneuploid tumour. Mixed mucinous carcinomas had a DNA content resembling that of common ductal carcinoma with 11 aneuploid tumours. Aneuploid tumours tended to be of higher grade and stage than diploid tumours. The survival of patients with pure mucinous carcinoma was better than that of patients with mixed mucinous carcinoma. Mucinous carcinoma should be classified as such only if it is a pure mucinous carcinoma.