Most urologists perform adjuvant radiation therapy for stage 1 (TxN0M0) testicular seminoma after orchiectomy, although the majority of patients with clinical stage 1 seminoma do not have occult metastases and therefore do not require elective nodal irradiation. However, there are currently no clinical or histological parameters that can be used to distinguish patients who need radiation therapy from those who do not. We reported previously that estimates of volume-weighted mean nuclear volume (MNV) were a better predictor of the prognosis of prostate cancer and renal cell carcinoma than subjective histological grading. Here, we examined the usefulness of estimation of MNV for predicting the prognosis of primary testicular seminoma. A retrospective study of 57 patients with testicular seminoma diagnosed between April 1981 and March 1997 at Kobe City General Hospital was performed. Unbiased estimates of MNV data were compared for prognostic value with the level of beta-human chorionic gonadotropin (beta-HCG), alpha-fetoprotein (AFP), alkaline phosphatase (ALP), and lactate dehydrogenase (LDH). Fifty patients were stage 1 (TxNoMo), and 7 patients were stage 2 (TxN1-2M0). All patients received orchiectomy, followed by radiation therapy. Estimates of MNV of stage 2 patients were significantly larger than that of stage 1 patients (P = 0.0142). Although the LDH level was also significantly higher in stage 2 (P = 0.001), there were no significant differences between stages 1 and 2 with respect to beta-HCG (P = 0.997), ALP (P = 0.226), and AFP (P = 0.467). Multivariate logistic regression analysis revealed that the estimate of MNV was the only variable predicting lymph node metastasis (P = 0.0315). In stage 1 patients, only the estimate of MNV was significantly correlated with progression-free survival (P = 0.0118). These findings indicate that the estimate of MNV may be an important prognostic indicator for testicular seminoma. Estimates of MNV may also be useful for excluding patients from surveillance protocols.