There is some evidence for a genetic influence on sexual orientation. However, gay men have fewer children than heterosexual men. Increased fecundity in the biological relatives of gay men could offset this selection pressure. We measured family size in gay (n = 301) and heterosexual (n = 404) men, attending clinics for sexually transmitted infections. The main outcome measure was the number of each man's uncles and aunts, first cousins, siblings, nephews and nieces, and his own children. With the exception of the participants' own offspring, mean family size of each category of relatives was significantly larger for gay men (paternal and maternal total OR = 1.02, CI = 1.01-1.03). This remained the case after adjustment for other predictors of family size (paternal and maternal total OR = 1.02, CI = 1.00-1.03). We found increased fecundity in the relatives of gay men and this is one explanation of how a genetic influence might persist in spite of reduced reproductive fitness in the gay phenotype. There are, however, a number of alternative explanations for our finding, including unknown psychological and social factors, which might mediate the association between family size and sexual orientation.