We analyzed 92 clinical stage I nonseminomatous testicular germ cell tumors for primary tumor histological factors that would distinguish true pathological stage I disease (N = 54) from those patients who harbored occult disease and actually were later found to have pathological stage II disease (N = 38). Primary tumor pathological material was analyzed for vascular invasion, lymphatic invasion, tunical invasion, and quantitative determination of percentage of the primary tumor composed of embryonal carcinoma, yolk sac carcinoma, teratoma, and seminoma. Univariate logistic regression analyses revealed that vascular invasion (P = 0.0001), percentage of embryonal carcinoma (P = 0.0001), lymphatic invasion (P = 0.0001), and tunical invasion (P = 0.0013) were higher in pathological stage II and that percentage of teratoma (P = 0.0001) and of yolk sac carcinoma (P = 0.0174) were higher in stage I. Percentage of seminoma was not significant. Individually, these parameters were able to correctly predict occult disease 66.3 to 80.4% of the time. In multivariate logistic regression analysis, only vascular invasion and percentage of embryonal carcinoma remained significant, and a model using these two variables was able to correctly predict stage 85.9% of the time. Vascular invasion and determination of percentage of embryonal carcinoma should be assessed for all clinical stage I nonseminomatous germ cell tumor patients and the model presented herein can be used clinically to predict the likelihood of occult disease and dictate therapy.