A 20 year old male patient presented with lack of sexual development. On examination he was eunuchoidal and hypogonadal, and olfactory function testing showed he was anosmic. Biochemical investigations proved he was hypogonadotrophic. Kallmann's syndrome was therefore diagnosed. His appearance was very different from his alleged identical twin who had undergone a normal puberty and had normal plasma testosterone and gonadotrophin levels. However, the twin was hyposmic. Genetic fingerprinting confirmed the twins were identical. Why Kallman's syndrome was incompletely expressed in one of them is unexplained. The parents and a normally menstruating sister had normal olfactory function.