The combination of mitozantrone and prednimustine has been reported to elicit response rates of around 50% in patients with advanced breast cancer. In the present trial, either three or nine courses of this combination were given to previously untreated patients with advanced breast cancer. Mitozantrone was given at 12 mg/m2 on day 1 and prednimustine was given orally at 130 mg/m2 on days 1-5; treatment was repeated every 4 weeks. A total of 34 patients were treated; the performance status was 0-1 in 29 subjects and 2 in 5 cases. Locoregional disease only was present in 13 patients; 9 showed lung involvement; 8, liver; 3, bone; and 1, stomach involvement. A total of 10 subjects had received no prior hormone therapy. The median disease-free interval from the time of initial diagnosis was 24 months (range, 0-144 months). In all 14/23 patients exhibited an oestrogen receptor level of greater than 20 fmol. Grade 1 nausea and vomiting occurred in 16 patients and that of grade 2-3, in 11 subjects; nausea was prolonged for greater than 10 days in 7 cases. Grade 4 neutropenia occurred in 2 patients. The response rate was 21% (95% confidence interval, 8%-38%). The combination of mitozantrone and oral prednimustine is toxic and displays low activity.