The index "the proportion of a cohort that have become delinquent by a given age," i.e., "prevalence of delinquency," is an important social indicator. This article discusses methods by which this index can be estimated from data, and corrects errors in previous sex- and race-specific prevalence estimates published by T.P. Monahan (1960) for the city of Philadelphia. The authors conclude that the difference between the sexes and between the races shown by these corrected prevalence estimates are of sufficient magnitude to render suspect any comparisons of prevalences of delinquency among cohorts which do not take account of the sex and race compositions of the cohorts to be compared. The article discusses the concept of prevalence generally, clarifies the differences between various rates that are employed in the calculation of prevalence, and corrects Monahan's error, determined to be substantial in the case of Negro boys. Corrections are given for prevalence estimates in Monahan's most important series of data, the composite for the years 1949-54, and suggestions are made as to how these might be applied to the remainder of his data, for individual years, if these results are required. Tables, references