Four sibs, 2 males and 2 females, were found to have the Cohen syndrome. All had moderate mental retardation, microcephaly, hypotonia, and narrow hands and feet with elongated fingers and toes; 3 were short of stature (2.0-3.5 SD below the mean) with weight between 10th and 50th centile and truncal obesity. Most of the facial characteristics of the syndrome were present: exotropia, prominent ears, short philtrum, and high nasal bridge. Each manifestation varied in severity from one sib to the other. The younger girl also had rheumatoid arthritis. Mild delay of puberty was described in 3 of the sibs. However, one of them has delivered a male infant with normal appearance whose psychomotor development has been normal (as of 9 months). No endocrine problems were documented in the sibship. All patients had normal chromosomes. The data on this sibship support the hypothesis of autosomal recessive inheritance of the Cohen syndrome. Microcephaly and short stature should be stressed as frequent manifestations of the syndrome. The variable expressivity, even among sibs, may be responsible for the paucity of reports on the mildest forms of the Cohen syndrome.