Objective: To assess the predictive value of undulant alpha fetoprotein (AFP) course in recurrence of germ cell tumors (GCT). Patients/method: A retrospective file search of 491 patients with GCT was performed and 18 patients who had an undulant course of AFP levels (increased and spontaneously decreased) without any demonstrable tumor on radiological images were selected for the study. Results: The median age of the 10 boys and 8 girls was 2 years (1.1-16 years). All the patients were disease-free for a median of 16.2 months (8.4-132 months). At follow-up, radiologically proven tumor recurrences were diagnosed in 8 patients in median 12.2 3.2 months from the initial diagnosis (8.4-18.2 months) and 7.4 2.2 months from the last normal level of AFP (5.0-11.2 months). The mean peak AFP values within the first 90 days were 24 38.1 ng/mL and 11.1 10.9 ng/mL in patients who did and did not have recurrent disease, respectively (p = .0051). The patients whose AFP levels were higher than 10 ng/mL were likely to have recurrent disease (p = .02). Conclusion: Without a demonstrable tumor, the small amplitude undulations may only require careful follow-up. However, it should be kept in mind that despite the decrease in AFP levels, there still might be a risk of recurrence, especially in patients who have greater amplitude undulations.