The aim of the study was to compare efficacy and tolerability of the new aromatase inhibitor formestane (Lentaron) with megestrol acetate (Megestat) (MGA) in postmenopausal patients with advanced breast cancer. 179 patients were randomised to receive either 250 mg formestane intramuscularly biweekly or MGA 160 mg orally daily. 51% of the patients had received tamoxifen as adjuvant treatment; 73% of the patients had positive and 16% unknown oestrogen receptor values. The response rate was 17% in both treatment arms (95% confidence interval 10-26% for formestane and 10-27% for MGA). Disease stabilisation > or = 6 months was seen in 25% of the formestane and 22% of the MGA patients. Time to treatment failure was 120 days in the formestane arm and 111 days in the MGA arm. There was no significant difference between the treatments with regard to response rate and time to treatment failure. Overall toxicity was similar in both arms, but weight gain > 3 kg (P = 0.081) and severe cardiovascular toxicity (P = 0.044) were more frequently observed with MGA, e.g. deep vein thrombosis 0/90 formestane versus 5/81 MGA cases (P = 0.022). Formestane was associated with worsening of hot flushes/sleeping problems (P = 0.051) and mild leucopenia (P = 0.004). In our study, formestane and MGA showed similar antineoplastic activity as second-line hormonal treatment for advanced breast cancer. Both drugs have a specific toxicity profile. MGA was associated with significantly more severe cardiovascular toxicity and weight increase than formestane.