Linkage of at least two complementation groups of ataxia-telangiectasia (AT) to the chromosomal region 11q23 is now well established. We provide here an 18-point map of the surrounding genomic region, derived from linkage analysis of 40 CEPH families. On the basis of this map, 111 AT families from Turkey, Israel, England, Italy, and the United States were analyzed, localizing the AT gene(s) to an 8-cM sex-averaged interval between the markers STMY and D11S132/NCAM. A new Monte Carlo method for computing approximate location scores estimates this location as being at least 10(8) times more likely than the next most likely interval, with a support interval midway between STMY and D11S132 that is either 5.2 cM (sex-averaged and conservatively based on 3 lod scores from the maximum-location score) or 2.8 cM (male specific, based on a 2.72:1 interval-specific female-to-male distance ratio.