We report the distribution of a previously described 9.1-kb insertion-deletion polymorphism located on chromosome 22. We analyzed 1,844 individuals sampled from 26 Mediterranean populations in mainland Italy, Sicily, Sardinia, Tunisia, Libya, Morocco, Egypt, Greece, and Albania. The 9.1 kb - allele is the prevalent allele in the North African (range, 0.53-0.56), Greek (0.51), and Albanian (0.66) populations, whereas the 9.1 kb + allele is most frequent in a mainland Italian town (0.55) and in all Sicilian and Sardinian towns and villages thus far tested, with marked fluctuation ranges of 0.53-0.78 and 0.56-0.80, respectively. In tests for Hardy-Weinberg equilibrium the genotype frequencies observed in Athens and in four of the nine towns in Sicily (but in none of the towns in Sardinia) departed highly significantly from the expected values. Identical results were found in the same towns for a second insertion-deletion polymorphism located on chromosome 22q13 at a distance compatible with a low incidence of recombination. The data, which are in good agreement with the different histories of the two islands (Sardinia and Sicily), are consistent with a West-east differentiation in Sicily and support the evidence for ancient gene flow from the Iberian peninsula to Sardinia.