Seminomas account for 50% of testicular germ-cell tumors, and more than 90% of these are classic seminomas. When patients with a histologically pure testicular seminoma show an elevated level of serum á-fetoprotein (AFP), it is generally assumed that an undetected focus of yolk sac tumor (YST) is present and the patient is managed with a treatment regimen for non-seminomatous tumor. We studied 10 cases of histologically pure seminoma with elevated levels of serum AFP in an attempt to identify any distinctive clinical, histopathologic, or immunohistochemical features. The patients ranged in age from 27 to 48 years (mean, 31 years). Eight patients had primary tumors of the testis, and two presented with supraclavicular and ileal tumors. The clinical stage at presentation varied: four tumors were stage I, four were stage II, and two were stage III. Serum levels of AFP were elevated in all patients at ranges of 10.4 to 16 ng/ml (mean, 12.0 ng/ml). In all patients, the primary tumors and metastases when present exhibited classic seminoma histology without other germ-cell components. The tumor cells expressed keratin in seven cases. The pattern of keratin immunoreactivity ranged from focal staining in five cases to moderate staining in two cases. All cases were negative for AFP, and the nine cases in which staining for CD30 (Ki-1) was performed were also negative. All four patients with stage I tumors underwent the conventional therapy for pure seminoma, i.e., orchiectomy and subsequent radiation therapy. Five patients received treatment for non-seminomatous tumors, i.e., chemotherapy after orchiectomy. Extensive work-up failed to detect the primary tumor in one patient, and he was treated for a non-seminomatous tumor, undergoing chemotherapy and irradiation. All patients are alive and well, and none has developed evidence of YST at a mean follow-up of 6 years (range, 6 months to 10 years). However, one patient who presented with an ileal metastasis recently developed a second primary extragonadal mediastinal mixed germ-cell tumor with YST and embryonal carcinoma components and an elevated serum level of AFP (27,000 ng/ml) after a 10-year disease-free follow-up. This study strongly suggests that minor elevations (