Refinement of an "index" marker genetic linkage map of human chromosome 9 using the CEPH reference pedigrees has been achieved through the addition of 11 markers to the previous map of 26 markers. Five of the 11 markers added to the map are new markers of the GATA repeat type, 1 is a complex repeat, and the remaining 5 as well as the original 26 markers are all GT/CA repeats. Twelve definite and five probable mutations were detected in this analysis and were more common for the GATA repeats than the GT/CA repeats. Strong evidence for positive interference was seen over the length of the chromosome, but there were significantly more double recombination events in the pericentromeric region than elsewhere, suggesting that interference is less strong in that region.