In cases of disputed parentage, it is commonly thought that the probability of paternity (P) represents the probability that the tested man is the biologic father of the child whose parentage is in doubt and that the paternity index (PI) is the likelihood ratio that enters into this probability. Using the results of HLA typing in a sample of disputed parentage cases, the distribution of P was examined within two groups: actual mother-alleged father-child trios, and simulated mother-unrelated man-child trios. These distributions were found to be roughly consistent with the usual understanding of P.