OBJECTIVE To define the prevalence of low-level sex chromosome mosaicism in a cohort of infertile men. METHODS Prospective cohort study of infertile men. METHODS Tertiary university infertility center. METHODS One hundred one consecutive men who presented with primary infertility for evaluation. METHODS Fluorescent in situ hybridization for X and Y was performed on 200 cells, and if an aberrant sex chromosome complement was noted, 400 cells were counted. For this study, any abnormality in sex chromosome complement was defined as micromosaicism. METHODS Low-level sex chromosome mosaicism. RESULTS Sixty-seven of these men (67%) had no mosaicism, and 34 men (34%) had micromosaicism. The median percentage of abnormal chromosomes in these men was 2%. The mean age of the men without micromosaicism was lower than for men with micromosaicism (31.1 years vs. 35.2 years). A trend toward higher FSH levels in men with low-level mosaicism was seen. Median sperm density and percent motility were higher in normal men. Percent normal morphology was identical between groups. CONCLUSIONS We found low-level sex chromosome mosaicism in 34% of infertile men who presented for evaluation. Men with low-level mosaicism were significantly older. Low-level mosaicism may emerge with advancing age and may therefore help to explain the decline in fertility potential seen in older men.