The effects of weight loss on erectile function and hormones have not been well studied. The aim of this study was to measure the degree to which sexual function and in particular erectile function and hormonal environment change after substantial weight loss, surgically and non-surgically induced in the morbidly obese male in a prospective randomized long-term controlled trial. Furthermore, how surgery makes a difference when treating morbidly obese men was envisaged in this context. We prospectively studied 20 morbidly obese men for 24 months, divided into two groups: group A included 10 patients who underwent life style modifications (exercise and diet) for 4 months and subsequently gastric bypass, and another 10 patients in group B were kept on weekly follow-up. None of the men were taking phosphodiesterase type-5 inhibitors. All patients underwent International Index of Erectile Function (IIEF)-5 questionnaire, serum oestradiol, prolactin (PRL), luteinizing (LH) and follicle-stimulating (FSH) hormones, free and total testosterone (FT and TT) at baseline (time 0), surgery - 4 months latter baseline (time 1) and final evaluation - 24 months (time 2). From times 0 to 1, group A presented a mean body mass index (BMI) reduction of 12.6 (p < 0.0001), whereas group B, 2.1 (p > 0.05). The BMI reductions between times 0 and 2 were 24.7 (p < 0.0001) and 0.7 (p > 0.05) for groups A and B respectively. BMI average between the two groups was similar at time 0 (p = 0.2142), and different at times 1 (p = 0.0033) and 2 (p < 0.0006). Increase in IIEF-5 score (p = 0.0469), TT (p = 0.0349) and FSH levels (p = 0.0025), and reduction in PRL level (p < 0.0001) were observed in group A from times 0 to 2 and 1 to 2. There were no changes from times 0 to 1. Comparing groups A and B at time 2, IIEF-5, TT and FT increased significantly in group A (p = 0.0224, 0.0043 and 0.0149 respectively). Surgery-induced weight loss increased erectile function quality measured by IIEF-5 questionnaire, increased TT, FT and FSH and reduced PRL levels. The hormonal impact verified could justify the improvement in erectile function. Lifestyle modifications impacted BMI without hormonal or sexual impact in morbidly obese. New studies are warranted in the field to support our data.