Testicular germ cell tumors (TGCT) are the most common cancer among young men in the United States and Western Europe. Prior evidence suggests that TGCT may arise in perinatal life, although few risk factors have yet been identified. To study the etiology of TGCT, the US Servicemen's Testicular Tumor Environmental and Endocrine Determinants (STEED) case-control study enrolled participants and their mothers between 2002 and 2005. Five hundred twenty-seven mothers of cases and 561 mothers of controls provided information on perinatal variables. Logistic regression was used to calculate odds ratios (OR) and 95% confidence intervals (95%CI) associated with the candidate risk factors. Analyses were conducted for all TGCT together and for each histologic subgroup (seminoma and nonseminoma) separately. Young maternal age (<20 vs. 20–29 years, OR = 1.51, 95%CI: 1.09–2.10), young paternal age (<25 vs. 25–29 years, OR = 1.45, 95%CI: 1.08–1.94), maternal parity (3 vs. 1, OR = 0.63, 95%CI: 0.44–0.90) and breech birth (OR = 1.92, 95%CI: 1.03–3.56) were associated with risk of TGCT. For seminoma, young maternal age (<20 vs. 20–29 years, OR = 1.67, 95%CI: 1.10–2.54), young paternal age (<25 vs. 25–29 years, OR = 1.53, 95%CI: 1.03–2.27), maternal parity (3 vs. 1, OR = 0.58, 95%CI: 0.35–0.96) and low birth weight (<2,500 g vs. 2,500–4,000 g, OR = 1.82, 95%CI: 1.00–3.30) were risk factors. Nonseminoma was associated with breech birth (OR = 2.44, 95%CI: 1.25–4.78) and Cesarean section (OR = 2.10, 95%CI: 1.25–3.54). These results support the hypothesis that TGCT may originate in very early life. © 2008 Wiley-Liss, Inc.