A previously unrecognised X-chromosomal mental retardation syndrome is described. Clinical hallmarks are mental retardation, epileptic seizures, hypogonadism, and -genitalism, microcephaly and obesity. Life expectancy of patients is less than two years. Based on the major clinical symptoms this condition is referred to by the acronym MEHMO. Haplotype and two-point linkage analyses in a large three-generation family assign the disease locus to Xp21.1-p22.13, to a region that is flanked by CYBB and DXS365.