Multiple animal studies, few clinical case reports and one study have observed decreased testosterone production and gynaecomastia as adverse effect of imatinib therapy. We have prospectively studied testosterone, LH and FSH levels at baseline and at 6 months of imatinib treatment in 34 newly diagnosed male BCR-ABL positive CML patients. While none of the patients had gynaecomastia at 6 months, the proportion of patients with low testosterone level increased significantly from 11.8% at baseline to 58.8% (p < 0.001) and those with high LH and FSH increased significantly from 26.4% and 23.5% to 82.4% and 76.4%, respectively (p < 0.001 and p < 0.001). Serum testosterone levels decreased significantly (p = 0.002) and serum LH and FSH levels increased significantly at 6 months of imatinib therapy (p = 0.001 and p = 0.003) in comparison to baseline levels. The findings document the effect of imatinib on testosterone levels in adult CML patients much before than reported earlier.