BACKGROUND Carboplatin is an analogue of cisplatin with less nonhematologic toxicity than the parent compound. It has been demonstrated previously to have activity against a spectrum of pediatric brain tumors. This Phase II study was undertaken to assess the activity of carboplatin in children with various solid tumors. METHODS Between October 1985 and March 1988, the Childrens Cancer Group entered 117 patients with drug-resistant, recurrent lymphomas and solid tumors, excluding primary central nervous system tumors, into a Phase II trial of carboplatin given intravenously at a dosage of 560 mg/m2 over 1 hour every 4 weeks. RESULTS A complete response was seen in 1 of 15 evaluable patients with Ewing's sarcoma. Partial responses were seen in 2 of 17 evaluable patients with neuroblastoma, 1 of 16 with soft tissue sarcoma, 2 of 5 with Wilms' tumor, and 1 with an endodermal sinus tumor of the testis. Objective responses were not seen in patients with malignant lymphoma, osteosarcoma, or hepatoma. Four of 7 patients who responded to carboplatin had previously received cisplatin. Sixty-two percent of patients had a platelet count nadir of less than 50,000/mm3, and 41% had an absolute neutrophil count nadir of less than 1,000/mm3. Ototoxicity, nephrotoxicity, hypomagnesemia, hypertransaminasemia, and nausea and vomiting each were seen in fewer than 10% of patients. CONCLUSIONS Carboplatin has some activity against Wilms' tumor, Ewing's sarcoma, neuroblastoma, soft tissue sarcoma, and endodermal sinus tumor of the testis. Activity was not demonstrated against osteosarcoma, malignant lymphoma, hepatoma, and miscellaneous other tumors. Myelosuppression was seen commonly, and nonhematologic toxicity was infrequent.