This report deals with a randomized prospective multicentric clinical trial in childhood rhabdomyosarcoma (RMS) conducted to evaluate the toxicity and the effectiveness of dactinomycin (ACT-D) administered as high, single doses v five-day, divided doses administered in combination with vincristine (VCR) and cyclophosphamide (CYC). Fifty-five group III evaluable patients (pts) less than 15 years of age with tumor size greater than 5 cm in diameter, without high-risk features of CNS involvement, and 15 group IV RMS pts were randomized to receive VAC as primary chemotherapy (CT): VCR, 1.5 mg/m2 intravenously (IV) days 1 and 8; CYC, 275 mg/m2 IV days 1 through 5; and ACT-D, 0.45 mg/m2 IV days 1 through 5 every 28 days for three cycles (33 pts), or VAC-M: CYC, 150 mg/m2 intramuscularly (IM) days 1 through 7; VCR, 2.0 mg/m2 IV day 8; and ACT-D, 1.7 mg/m2 IV day 8 every 21 days for four cycles (37 pts). Major responses (complete plus partial responses [PR]) were obtained in 67% of the VAC pts and in 70% of the VAC-M pts. Toxic effects were low, and no increased toxicity was observed in pts treated with high, single-dose ACT-D. These results confirm the effectiveness and feasibility of single, high doses of ACT-D with the advantage of requiring less pt hospitalization.