Eighty-one patients with advanced testicular cancer were evaluated for gynecomastia or severe breast tenderness at diagnosis and after platinum-based chemotherapy. The prognostic significance of gynecomastia in these two settings was explored. At presentation, 10% (8 patients) had gynecomastia or breast tenderness and elevated HCG levels. The likelihood of gynecomastia was greater with increasing HCG level (P = 0.002). However, gynecomastia at presentation was a more powerful independent discriminant of poor survival than the initial HCG level by multivariate analysis (P = 0.004). Fifteen percent (12 patients) developed transient gynecomastia after chemotherapy not attributable to other known causes. HCG levels were normal. Endocrine evaluation typically revealed elevated FSH, LH, and estradiol/testosterone ratios. This may have reflected damage to testicular germinal epithelium. All 12 patients are alive without disease in contrast to the 8 patients who had gynecomastia at diagnosis. Therapy decisions should therefore be based on the time of onset of gynecomastia and in the context of appropriate clinical markers and evaluation.