The response to a single course of carboplatin has been investigated in 12 patients with previously untreated non-seminomatous testicular germ cell tumours. Patients received one course of carboplatin at a dose calculated to achieve a target area under the free carboplatin plasma concentration versus time curve (AUC) of 7 mg/ml x mins using the formula: dose (mgs) = target AUC x (GFR + 25). Response to carboplatin was assessed after a single course and treatment was then continued on the POMB/ACE schedule. Ten of 12 patients had either a greater than 50% decrease in serum HCG and/or AFP levels or a greater than 50% decrease in tumour volume after a single course of carboplatin. No patient had evidence of disease progression after carboplatin. This study demonstrates that single agent carboplatin is highly active in patients with non-seminomatous testicular germ cell tumours and thus provides evidence to justify its inclusion in chemotherapy combinations.