4'-O-Tetrahydropyranyl adriamycin (THP adriamycin) is a new anthracycline active as a single drug in advanced breast cancer. We have undertaken a phase II study as first-line treatment for metastatic disease with THP adriamycin day 1 = 40 mg/m2 i.v. bolus and 5-fluorouracil day 1 to day 5 = 750 mg/m2 as a continuous i.v. infusion. The dose of THP adriamycin was further escalated up to the maximal tolerated dose defined as grade 3 granulopenia for each patient. Thirty-nine patients were included, 37 being so far evaluable for toxicity and for efficacy. The mean number of cycles given was 5 (range: 2-12). The overall response rate (CR + PR) was 54% (95% CI: 37.9-70.1) and the CR rate 8%. Sites of response were as follows: lung 6/9, liver 11/18, breast 4/8, nodes 7/14, skin 3/8, bone 2/8. Neutropenia with grade 3 + 4 nadir values was observed in 70.2% of the patients according to the objective of the study. No severe thrombopenia or anemia occurred. Stomatitis grade 3 was seen in 27% and grade 4 in 3% of the patients. Alopecia grade 2 was seen in 18% and grade 3 in 9%. No other toxicity was observed. We conclude that this association is effective in metastatic breast cancer, giving few alopecia. A high response rate in liver metastases warrants further evaluation.