In this prospective study, patients with "high risk' primary MDS, namely RAEB or RAEBt, were treated with combination chemotherapy (CT) supported by GM-CSF. The induction CT consisted of idarubicin 6 mg/m2 days 1-3 and cytosine-arabinoside 200 mg/m2 in 12 h infusion, days 1-5. The GM-CSF 3 micrograms/kg s.c. was given on day 6 until the neutrophil count was 1 x 10(9)/l. Postremission CT consisted of two similar courses. Patients not in remission after two courses of CT were considered as treatment failures. Twenty-two patients with a median age of 64 years, range 50-79 years (11 RAEB and 11 RAEBt) were evaluable. Twelve out of 22 patients (54.5%) achieved complete remission (CR) and four, partial remission. Six patients were resistant to treatment; there were two toxic deaths; seven patients achieved CR after the first course and five after two courses. The median time of neutrophil recovery to 1 x 10(9)/l was day 15 (range 3-22) after the first course of treatment and day 14 (range 4-21) after the second. Thirteen out of 22 patients developed febrile episodes after the first course of treatment and nine after the second. The median duration of CR was 12 months. The median survival for CR patients was 24 months, for non-CR patients, 12 months; while survival for the whole population was 18 months. In conclusion, the results of this study indicate that the administration of moderately intensive CT supported by GM-CSF in "poor risk' MDS gives promising results; the response rate is high for this disease, while the incidence of toxic death is low. GM-CSF appears to accelerate neutrophil recovery and probably reduces the incidence of infection.