A patient with molybdenum cofactor deficiency (producing the biochemical abnormalities associated with deficiencies of sulphite oxidase and xanthine dehydrogenase) clinically expressed Marfan-like habitus with dislocated lenses, vertebral abnormality, learning disability, moderate hemiplegia, increased medial lentiform MRI signal and intermittent microscopic haematuria. S-Sulphocysteine was present in plasma and urine, and the oxidized derivative of a molybdopterin precursor (precursor Z), together with xanthine and hypoxanthine, were elevated in urine. Blood uric acid was < 1 mg/dl, while urinary urothione was not detected. These data indicate a functionally inadequate terminal enzyme for converting precursor Z to active molybdopterin (complementation group B of general molybdenum cofactor deficiency). Although the biochemical parameters were indicative of a severe deficiency state, the patient has survived into the third decade with a less severe clinical spectrum than has generally been associated with this disease.