Reproduction rates in 294 men with severe hemophilia, 327 of their mothers, and 215 of their sisters were compared with Vital Statistics data for the general US population matched for exact age, calendar year between 1940 and 1977, and, for women, parity. Before 1950, reproduction rates in hemophiliacs and in mothers did not differ from those of the general population, but thereafter the rates fell notably. Births to sisters who were para 0 were lower than expected beginning in 1965, but births to sisters who had had one or more nonhemophiliac children were similar to those expected of the general population. The decline in fecundity may be caused by greater availability of genetic counseling and birth control measures and by social changes. Many prospective parents would welcome the opportunity to have unafflicted children but refuse selective abortion. Preconception gender selection would be appropriate for this sex-linked disorder.