Surgical treatments such as the Roux-en-Y gastric bypass operation result in the successful treatment of morbid obesity; however, this type of operation may cause long-term side effects due to the reduced absorption of nutrients. Here, we present data suggesting that this operation can result in secondary infertility in males. Six healthy, previously fertile male subjects presented in our centre for secondary infertility after a Roux-en-Y gastric bypass operation for morbid obesity. Reproductive function was assessed with a series of spermiograms, and by testicular biopsy. Secondary azoospermy with complete spermatogenic arrest was diagnosed. The results suggest that weight reduction surgery may cause reproductive dysfunction.