Variation in urinary creatinine concentration was studied in a Gambian community. Whilst there was appreciable day to day variation there were also differences between subjects and between different times of year. Day to day changes in creatinine concentration correlated with changes in egg count, which were, in proportion, smaller. While there are difficulties in the use of creatinine as a reference index, it is suggested that knowledge of creatinine concentrations may eliminate some of the ambiguities arising in the interpretation of Schistosoma haematobium egg counts in random, untimed urine specimens. The relation between the two variables and the extent of seasonal changes in creatinine concentration suggest that changes in the mean egg count of a group of subjects due to changes in urine flow are usually small and seldom of the magnitude which some have assumed.