Three boys from two families were identified as having a syndrome of X-linked mental retardation (XLMR) with microcephaly and short stature, clinically resembling Renpenning syndrome but with normal size of testicles in affected men. When the effort to map the gene for the above condition was initiated, it was realized that the two families were actually related to each other. Over 50 polymorphic markers of known locations along the X chromosome were scored in this family in a study to map the disease gene. Nine affected and four unaffected males were genotyped to produce a maximum LOD score of 4.42 at zero recombination with markers in proximal Xq. The results indicate that the gene responsible for this disorder is located in the cytogenetic Xq12 to Xq21.31 interval of the X chromosome within a section of chromosome of about 17 cM between the AR and DXS1217 loci over some 25 mb. Since the gene for the X-linked mental retardation from the original Saskatchewan family described by Renpenning [Renpenning et al., 1962: Can Med Assoc J 87:954-956; Fox and Gerrard, 1980: Am J Med Genet 7:491-495] was recently mapped to a different nonoverlapping region [Stevenson et al., 1998: Am J Hum Genet 62:1092-1101] this would appear to be a separate disorder.