Aclarubicin, discovered by Umezawa in 1975, is a new cytostatic anthracycline antibiotic. It is one of the anthracyclines with the lowest cardiotoxicity, it is not mutagenic and it stimulates differentiation of tumour cells. The therapeutic index of aclarubicin (efficacy related to toxicity) is higher than that of doxorubicin and daunorubicin, using a proper dose schedule. Single dose therapy of aclarubicin shows only marginal efficacy, whereas multiple divided dose therapy exhibits efficacy comparable to that of doxorubicin and daunorubicin. Thus for clinical trials two dose schedules were designed: 25 mg/m2/day, days 1-7 for acute leukaemia; and 30 mg/m2/day, days 1-4 for solid tumours. Aclarubicin was shown to be highly active in acute leukaemia with 58% complete remissions in first relapse of AML. Good results were also seen in acute leukaemia in combination with cytosine arabinoside and thioguanine. In clinical trials with breast cancer and thyroid cancer the efficacy was in the same range as would be expected for doxorubicin, but side-effects were markedly reduced. Anorexia, mild nausea and infrequent vomiting were observed. Myelosuppression was common but dose reduction was not necessary. There was no alopecia and no congestive heart failure.