Studies in animals suggest that the bispiperazinedione ICRF-187 can prevent the development of dose-related doxorubicin-induced cardiac toxicity. In a randomized trial in 92 women with advanced breast cancer, we compared treatment with fluorouracil, doxorubicin, and cyclophosphamide (FDC), given every 21 days, with the same regimen preceded by administration of ICRF-187 (FDC + ICRF-187). Patients were withdrawn from the study when cardiac toxicity developed or the cancer progressed. The mean cumulative dose of doxorubicin tolerated by patients withdrawn from study was 397.2 mg per square meter of body-surface area in the FDC group and 466.3 mg in the FDC + ICRF-187 group (no significant difference). Eleven patients on the FDC + ICRF-187 arm received cumulative doxorubicin doses above 600 mg per square meter, whereas one receiving FDC was able to remain in the study beyond this dose. Antitumor response rates were similar (FDC vs. FDC + ICRF-187, 3 vs. 4 complete responses; 17 vs. 17 partial responses; and 9.3 vs. 10.3 months to disease progression). Although myelosuppression was slightly greater in the FDC + ICRF-187 group, the incidence of fever, infections, alopecia, nausea and vomiting, or death due to toxicity did not differ between the groups. Cardiac toxicity was evaluated by clinical examination, determination of the left ventricular ejection fraction by multigated nuclear scans, and endomyocardial biopsy. In comparisons of the FDC group with the FDC + ICRF-187 group, clinical congestive heart failure was observed in 11 as compared with 2 patients; the mean decrease in the left ventricular ejection fraction was 7 vs. 1 percent when the cumulative dose of doxorubicin was 250 to 399 mg per square meter (P = 0.02), 16 vs. 1 percent at 400 to 499 mg (P = 0.001), and 16 vs. 3 percent at 500 to 599 mg (P = 0.003); and the Billingham biopsy score was 2 or more in 5 of 13 patients undergoing biopsy vs. none of 13 (P = 0.03). We conclude that ICRF-187 offers significant protection against cardiac toxicity caused by doxorubicin, without affecting the antitumor effect of doxorubicin or the incidence of noncardiac toxic reactions.