To prospectively assess the role of the MDR1 gene in breast carcinomas, MDR1 RNA levels of breast carcinoma specimens were determined by slot blot analysis. In 59 evaluable patients with primary breast carcinomas, MDR1 RNA levels of the carcinomas were negative in 54%, low in 29% and high in 17% of the patients. No differences in age, menopause status, oestrogen and progesterone receptor levels, tumour size, lymph node involvement and c-erbB-2/neu gene expression were observed between MDR1 RNA negative patients and MDR1 RNA positive patients.