Chemotherapy-treated patients with advanced Hodgkin's disease (HD) differ considerably in acute hematotoxicity. Hematotoxicity may be indicative of pharmacological and metabolic heterogeneity. We hypothesized that low hematotoxicity might correlate with reduced systemic dose and thus reduced disease control. A total of 266 patients with advanced HD treated with cyclophosphamide, vincristine, procarbazine, prednisone, doxorubicin, bleomycin, vinblastine, and dacarbazine (COPP-ABVD) were analyzed (HD6 trial of the German Hodgkin's Lymphoma Study Group). The reported WHO grade of leukocytopenia was averaged over chemotherapy cycles given and weighted with the reciprocal dose intensity of the corresponding cycle. The low and high toxicity groups were defined in retrospect as having had an averaged WHO grade of leukocytopenia </=2.1 and >2.1, respectively. The independent impact of low hematological toxicity on freedom from treatment failure (FFTF) was assessed multivariately adjusting for the international prognostic score for advanced HD. The results were validated in two independent cohorts [181 patients treated with COPP-ABVD (HD9-trial) and 250 patients treated with COPP-ABV-ifosfamide, methotrexate, etoposide, and prednisone (IMEP) (HD6 trial)]. The 5-year FFTF rates were 68% for patients with high toxicity vs 47% for patients with low toxicity [multivariate relative risk (RR) 2.0, 95% confidence interval (CI) 1.4-3.0, p=0.0002]. Patients with low toxicity received significantly higher nominal dose ( p=0.02) and dose intensity ( p<0.0001). This finding was confirmed in both validation cohorts (multivariate RR 2.1, 95% CI 1.2-3.8, p=0.01 and RR 1.5, 95% CI 1.01-2.26, p=0.04, respectively). Patients with low hematotoxicity have significantly higher failure rates despite higher doses and dose intensity. Hematotoxicity is an independent prognostic factor for treatment outcome. This observation suggests a strategy of individualized dosing adapted to hematotoxicity.