Varicoceles are associated with ipsilateral testicular atrophy in the adolescent. To determine if varicoceles are capable of causing progressive loss of fertility, we conducted a date-matched, retrospective study. We reviewed 2,989 patients evaluated for infertility at our institution from 1985 to 1990. A total of 285 (8.5%) patients were diagnosed with secondary infertility. A varicocele was identified as the cause of the patient's infertility in 177 (69%) men with secondary infertility. When matched by date to an identical number of men with primary infertility in whom 128 (50%) were infertile on the basis of a varicocele effect, the difference was significant (p < 0.0001). We conclude that a varicocele in some men is a progressive and not a static lesion resulting in the loss of previously established fertility. In addition, varicoceles are not only the leading cause of infertility in men with secondary infertility, but also occur with a greater frequency than in men with primary infertility.