The combination of X-linked mental retardation (XLMR) and neurological disorders occurs in a number of syndromes. Differential diagnosis mostly depends on clinical data and mapping of responsible genes by linkage analysis. We present a Belgian family with severe XLMR and a progressive neurological disorder with ataxia, spasticity and convulsions. Biochemical investigations, neuroimaging and neuropathology were normal. Linkage analysis pointed to region Xq27-28 as the probable locus for the genetic defect. The sequence of the L1CAM cDNA, a possible candidate gene, proved to be normal in the patients. This suggests the presence of a genetic factor on Xq27-28, different from L1CAM, which can lead to severe XLMR and a progressive neurological disorder.