Sex differences in childhood urinary-tract infection have been looked for by reviewing the medical records of all patients who were admitted to one medical centre during a certain time period. There were 240 patients; 26 males and 214 females, all under 14 years of age. The disease in boys-as compared with that in girls-was found to be characterized by an earlier onset, a shorter delay in diagnosis, and a shorter duration; but a higher frequency of malformations, a greater number of rehospitalizations, and a greater need for surgical intervention. Proteus infection was found more frequently in boys, while Enterococcus and Staphylococcus were more frequent in girls. The initial symptomatology did not show any significant sex difference except in late childhood where the non-specific symptoms were more common in females. No sex difference was noted with respect to the frequency of vesico-ureteric reflux or of bacteriuria without pyuria. It seems that the sex variable in this disease is worth considering in dealing with individual patients as well as in presenting data on a series of patients.