BACKGROUND Apart from a recognized association between extragonadal mediastinal germ cell tumors (GCT) and the occurrence of hematologic malignancies, the risk of developing second nongerminal solid tumors after the diagnosis or treatment of extragonadal GCT is unknown. METHODS Six hundred thirty-five consecutive patients with extragonadal GCT treated at 11 centers in the U.S. and Europe during the era of cisplatin-based chemotherapy (1975-1996) were included into a large database. These patients were evaluated for the occurrence of second malignancies. RESULTS No treatment-related leukemia was observed in 611 patients treated with chemotherapy. In 7 patients, second solid tumors were observed, resulting in a frequency of 1.86% (95% confidence interval [95% CI], 1.79-1.93%) after a median follow-up of 55 months (95% CI, 50-60 months) (annual incidence, 0.30% [95% CI, 0.14-0.59]). Four solid tumors (57%) developed in patients with primary mediastinal and 3 tumors (43%) developed in patients with retroperitoneal GCT. Three patients (43%) had a nonseminomatous and 4 patients (57%) had a seminomatous histology. Six patients had been treated with chemotherapy and one patient with radiotherapy. Six of 7 solid tumors (86%) had developed within 5 years and 7 of 7 solid tumors within 10 years of diagnosis. The median time period to the occurrence of neoplasia was 47 months (range, 9-145 months). Four cutaneous tumors were observed (melanoma, two patients; basal cell carcinoma, one patient; and squamous cell carcinoma, one patient); the other three tumors were angiosarcoma, nonsmall cell lung carcinoma, and colorectal carcinoma. The overall risk for developing a second tumor was not increased compared with an age-matched general population with a standard incidence ratio (SIR) of 1.49 (95% CI, 0.60-3.06). An elevated risk for skin tumors was observed in all extragonadal GCT patients (SIR, 4.00 [95% CI, 1. 09-10.24]), as well as in the subgroup of patients treated with chemotherapy (SIR, 5.33 [95% CI, 1.45-13.65]). CONCLUSIONS This analysis excludes an increased biologic risk of developing second solid malignancies in patients with extragonadal GCT except for the previously reported association between primary mediastinal nonseminoma and hematologic disorders. The overall risk of developing second malignancies in extragonadal GCT patients appears to be comparable to that in patients with primary testicular carcinoma. The incremental occurrence of skin malignancies in patients treated with chemotherapy should be investigated further.