SUMMARY A chronic form of tyrositiosis was diagnosed in a mentally retarded girl, aged 13. Concomitant findings were skin lesions of localized epidermolytic hyperkeratosis existing without remission from infancy. A diet low in phenylalanine and tyrosine was started to reduce the patient's blood tyrosine to a normal level. After 6 months of successful dietary control of blood tyrosine, an unexpected result was a complete remission of the skin lesions. The diet was abandoned after a total period of 14 months and the abnormality of tyrosine metabolism became immediately prominent again. Some 4 months later, the skin lesions have recurred. A possible relationship between the congenital skin disorder and the abnormal tyrosine metabolism in this case is considered. It is well known that some inborn errors of amino acid metabolism may be associated with skin lesions. The most cotmnon of the amitio-acidaemias, phenylketonuria (PKU) is characteristically associated with decreased pigmentation. Infantile eczema, dry skin and non-specific rashes are also frequent occurrences and sensitivity to sunlight may be a problem. In some adult phcnylketonuric subjects, eczema may persist in spite of topical treatment and, in our experience, such cases occasionally respond to a modified PKU diet and the lesions may heal in 4-5 weeks. In Hartnup disease, a pellagra-like skin rash aggravated by sunlight is typically associated with periods of ataxia, mental retardation and characteristic amino-aciduria. Skin signs in homocystinuria are mild, consisting of livcdo reticularis, a malar flush and light-coloured hair and skin. In some cases of tyrosinosis, mild hyperpigmentation has been noted. We wish to report an unusual skin lesion occurring concomitantly with elevated blood tyrosine and excretion of tyrosine and its derivatives in urine (tyrosyluria) in a mentally retarded girl diagnosed as a chronic form of tyrosinosis (Zaieski & Hill, 1973).