Short tandem repeats (STRs) are subjected to two kinds of mutational modifications: point mutations and replication slippages. The latter is found to be the more frequent cause of STR modifications, but a satisfactory quantitative measure of the ratio of the two processes has yet to be determined. The comparison of entire genome sequences of closely enough related species enables one to obtain sufficient statistics by counting the differences in the STR regions. We analyzed human-chimpanzee DNA sequence alignments to obtain the counts of point mutations and replication slippage modifications. The results were compared with the results of a computer simulation, and the parameters quantifying the replication slippage probability as well as the probabilities of point mutations within the repeats were determined. It was found that within the STRs with repeated units consisting of one, two or three nucleotides, point mutations occur approximately twice as frequently as one would expect on the basis of the 1.2% difference between the human and chimpanzee genomes. As expected, the replication slippage probability is negligible below a 10-bp threshold and grows above this level. The replication slippage events outnumber the point mutations by one or two orders of magnitude, but are still lower by one order of magnitude relative to the mutability of the markers that are used for genotyping purposes.