Forty-one advanced seminoma patients with normal biochemical markers and a complete or partial radiographic response after cisplatin-based chemotherapy had a complete reevaluation of all known sites of disease. Twenty-three patients had a residual mass, and in 14 the mass was greater than or equal to 3 cm. Nineteen patients with a residual mass, including 13 with a mass greater than or equal to 3 cm in diameter, had surgical excision or biopsy. Four patients had viable seminoma and one patient had teratoma; all five of these patients had residual masses greater than or equal to 3 cm. Four patients with a residual mass were observed without surgery. One patient with a residual mass greater than or equal to 3 cm progressed with biopsy-proven seminoma. Therefore, six of 14 patients (42%) with a residual mass greater than or equal to 3 cm had viable residual tumor. Eighteen patients had no residual mass after chemotherapy. Ten of these patients had surgery or biopsy; none had viable tumor, but two have relapsed. Eight patients were observed and none have relapsed. Advanced seminoma patients with a residual mass greater than or equal to 3 cm after chemotherapy are at high risk for residual viable tumor. Additional therapy is indicated for these patients. For patients with normal imaging studies or a residual mass less than 3 cm, close observation without surgery is generally possible.