Little attention has been given the existence of male infertility in hyperthyroidism and the mechanisms adversely affecting spermatogenesis therein. The chance presentation of three young thyrotoxic men within a short period of time allowed us to document the common findings in all three of reversibly impaired semen quality. The unexpected finding of elevated testosterone and gonadotropin levels in each case, with return of these values to normal on attaining euthyroidism, and the finding of maturation arrest in one patient prior to treatment of his thyrotoxicosis, allow some considerations of the altered physiology and spermatogenic defect and suggest a need for further attention to this disorder.