Using data on children hospitalized with measles in Copenhagen in 1915-1925, it was found that secondary cases infected at home by someone of the opposite sex had a relative case-fatality rate risk (RR) of 1.52 (95%, confidence interval (Cl): 0.80-2.88) compared with secondary cases infected by someone of the same sex. Males infected by a girl suffered more severely than when infected by another boy (RR = 2.46; 95% Cl: 1.00-6.04) whereas there was no difference for girls. In households with two cases of measles, case fatality was significantly higher in families with a boy and a girl infected at the same time compared with families with two boys or two girls (RR = 1.89; 95% Cl: 1.06-3.37). There was no difference in mortality in families with two boys or two girls having measles at the same time. These results strengthen previous observations from Guinea-Bissau that close contact with a child of the opposite sex increases the severity of measles infection.