Rett syndrome (RS; MIM 312750) is a severe neurological disorder affecting exclusively females. Its prevalence is about 1 in 10 000 female births, and it is a prominent cause of profound mental handicap in women. RS is caused by mutations in the X-linked methyl CpG-binding protein 2 (MECP2) gene. These mutations were initially thought to be lethal in males. However, MECP2 mutations are now frequently identified in mentally retarded male patients. The frequency of disease-causing MECP2 mutations in this population is between 1.3% and 1.7%. Surprisingly, MECP2 mutations in males are responsible for a wide spectrum of neurological disorders, ranging from mild mental retardation to severe neonatal encephalopathy. The aim of this review is to describe the nature of the MECP2 mutations identified in male patients to date and their associated phenotypes.