After surgery and initial treatment, 126 patients with nonseminomatous germ cell tumors of the testis were followed by estimation of serum human chorionic gonadotropin (HCG), alpha-fetoprotein (AFP), and pregnancy-specific beta-1-glycoprotein (SP-1). The median follow-up time was 39 months (range, 12-69 months). An evaluation was made of the sensitivity, specificity, and predictive value of these markers to detect tumor relapse (TR) and partial remission (PR) rates. Five of the 35 (14%) Stage I patients had a TR, and in 4 the TR was recognized early by increasing serum levels of HCG (four times) and AFP (one time). In Stage II and III disease 17 of the 91 (18%) patients had a TR or PR. In 15 of the 17 patients TR and PR were associated with elevated serum levels of HCG (nine times), AFP (seven times), and SP-1 (five times). Rising levels of HCG and/or AFP preceded a clinical confirmation of TR and PR by 4 to 8 weeks. Serum SP-1 did not add useful information. The combined use of serum HCG and AFP gives a sensitive indicator for TR and PR: 19 of 22 (86%) of such patients were recognized. The specificity (98% and 100%, respectively) and the predictive value of positive tests (87% and 100%, respectively) were high for both HCG and AFP.