Forty patients with retroperitoneal metastasis from nonseminomatous testicular cancer treated with chemotherapy were retrospectively studied to (1) evaluate the predictive value of mass size as detected by computerized tomography (CT) as an indicator for postchemotherapy surgery and (2) determine the factors that influence relapse. Patients received two further courses of chemotherapy after their serum biomarkers became normal and computed tomography indicated a complete response or presence of a residual but stable mass. We found that patients with initial metastases less than 2 cm had a low frequency (14%) of residual masses after chemotherapy, vs. 59% for those with masses of 2-5 cm and 75% for those with masses of greater than 5 cm (P = 0.03). Of 22 patients with primary embryonal carcinoma, three of seven (43%) with residual masses after chemotherapy had mature teratoma at surgery. Six patients had small (1-2 cm) residual abnormalities that were not removed, and three of these patients relapsed. In conclusion, increasing size of retroperitoneal metastasis by CT scan predicts for increased likelihood of a residual mass after chemotherapy; patients who have a residual mass greater than or equal to 1 cm require retroperitoneal lymphadenectomy after chemotherapy, whether the tumor histology is embryonal carcinoma or teratoma. The role of surgery for patients who have residual retroperitoneal masses less than 1 cm after chemotherapy could not be determined from our study.