We evaluated the effect fit of underwear had on sperm production in two healthy adult males in their early thirties. The subjects alternated from wearing tight fitting bikini type briefs to loose fitting boxer type briefs in an ABAB withdrawal design. Conditions lasted three months and were alternated twice resulting in a one year study. The initial condition for each subject was randomly determined with the laboratory technologist blind to the conditions. Four semen parameters were analyzed in this study: sperm density, total number of sperm, total number of motile sperm, and total number of motile sperm per hour of abstinence. The results showed the semen parameters gradually decreased in tight conditions and gradually increased during loose conditions. Although preliminary, the results support the commonly held but undocumented belief that the fit of men's underwear can influence sperm production.