A 15-year-old girl affected by autosomal recessive 3-M slender boned nanism (3-MSBN) was studied. The clinically normal parents, two other obligate and two probable heterozygotes for the 3-MSBN gene from an unrelated family were radiologically investigated. All except one probably heterozygote showed mild features of the 3-MSBN, mainly bone slenderness and prominent talus. These findings are interpreted as demonstrative of the heterozygotic expression of the 3-MSBN gene.