Adenocarcinoma of the breast is the most common cancer in women worldwide and its incidence is increasing in most countries. The present study was designed to evaluate the relationship between different clinical stages and menopausal status using the activity of phase I and II carcinogen-metabolising enzymes in breast cancer patients. Fifty breast cancer patients clinically categorized as stage I, II and III, and as pre- and postmenopausal were chosen for the study. The levels of cytochrome P450 and b(5) and the activity of glutathione S-transferase (GST), gamma-glutamyl transpeptidase (GGT), DT-diaphorase (DTD) and NADPH diaphorase in tumour tissues and adjacent normal tissues were estimated. Enhanced levels of cytochrome P450 and b(5) and phase II enzyme activity were observed in breast tumour tissues compared with the corresponding uninvolved adjacent tissues irrespective of clinical stage and menopausal status of the patients. The magnitude of the changes in phase I and II enzyme status was, however, more pronounced in stage I and in premenopausal patients than in stage II and III and postmenopausal patients respectively. Our results suggest that the balance between phase I carcinogen activation and phase II detoxification systems may play an important role in the development of breast tumours.