Although the etiology of polycystic ovary syndrome (PCOS) is still unclear, LH is considered to play a central role in its pathogenesis. An immunologically anomalous form of LH, with two point mutations in the LHbeta gene, has been recently described. This genetic variant of LH (v-LH), of wide geographic distribution, is functionally different from wild-type (wt) LH. To assess the role of the v-LH in PCOS, we analyzed its frequency in groups of PCOS patients from Finland, The Netherlands, the United Kingdom, and the United States. The LH status was determined by two immunofluorometric assays from a total of 1466 subjects. The carrier frequency of the v-LH allele in the whole study population was 18.5%, being highest (28.9%) in Finland and lowest (11.2%) in The Netherlands. In the individual countries, the frequency of v-LH was similar in obese and nonobese controls, but in The Netherlands and Finland, it was 5- to 7-fold lower in obese PCOS subjects compared with that in the other groups (2-4.5% vs. 10.3-33.3%; P < 0.05). A similar tendency was found in the United States (5.7% vs. 11.1-25.0%), but not in the United Kingdom. The overall high prevalence of v-LH in healthy women and women with PCOS suggests that it is compatible with fertility. The similar frequency of v-LH in healthy nonobese and obese women indicates that obesity per se is not related to the variant. In contrast, the lower frequency of v-LH in obese PCOS patients suggests that v-LH somehow protects obese women from developing symptomatic PCOS. However, the regional differences in this finding between patients with apparently similar diagnostic criteria emphasizes the multifactorial nature of this syndrome, and that its pathogenesis may vary according to the genetic background. Although the definitive role of v-LH in PCOS remains to be proven, its determination may improve the prediction of risk of PCOS, especially in obese women.