Objective. Systemic lupus erythematosus (SLE) is a systemic autoimmune disease that predominantly affects women. Despite isolated reports of patients with coexisting Klinefelter's syndrome (47,XXY) and SLE, no association of Klinefelter's syndrome with SLE or any other autoimmune disease has been established. The present study was undertaken to investigate the prevalence of Klinefelter's syndrome in a large population of patients with SLE. Methods. Sex chromosome genotyping was performed in 981 SLE patients, of whom 213 were men. A first group of 844 SLE patients from 378 multiplex families and a second group of 137 men with nonfamilial SLE were evaluated. In selected cases, chromosomes were enumerated by fluorescence in situ hybridization (FISH) and karyotyping in transformed B cell lines. Results. Of 213 men with SLE, 5 had Klinefelter's syndrome (1 in 43). Four of them were heterozygous at X markers, and Klinefelter's syndrome was confirmed by FISH and karyotyping in the fifth. An overall rate of 47,XXY of 235 per 10,000 male SLE patients was found (95% confidence interval 77-539), a dramatic increase over the known prevalence of Klinefelter's syndrome in an unselected population (17 per 10,000 live male births). Asking men with SLE about fertility was highly sensitive (100%) for Klinefelter's syndrome. All 768 women with SLE were heterozygous at X. Conclusion. The frequency of Klinefelter's syndrome (47,XXY), often subclinical, is increased in men with SLE by ∼ 14-fold compared with its prevalence in men without SLE. Diagnostic vigilance for 47,XXY in male patients with SLE is warranted. These data are the first to show an association of Klinefelter's syndrome with an autoimmune disease found predominantly in women. The risk of SLE in men with Klinefelter's syndrome is predicted to be similar to the risk in normal women with 46,XX and ∼ 14-fold higher than in men with 46,XY, consistent with the notion that SLE susceptibility is partly explained by an X chromosome genedose effect. © 2008, American College of Rheumatology.