A review of the literature does not show that minimal or mild endometriosis unquestionably causes infertility although its association is known; the cause only becomes apparent as the disease progresses. In these patients therefore, prolonged infertility for other reasons may have exacerbated or predisposed a tendency in them to develop the disease. Sperm dysfunction in their partners may have been one factor. Sperm penetration assays using mid-cycle (estrus) bovine cervical mucus were studied in 22 men whose wives were confirmed to have minimal or mild endometriosis. All men had normal semen analyses. For controls, 16 men of proved fertility were used. Whilst all but one (94%) of the fertile men showed adequate penetration (>30 mm), only 59% of the men in the study group achieved this penetration (p < 0.05). These findings provide evidence that minimal or mild endometriosis may not be the primary cause of infertility in patients in whom the diagnosis is made.