It is reported on a female patient with a classical homocystinuria who showed all typical symptoms of the cystathionine-synthesis-insufficiency, such as tall stature, phacetomy, arachnodactyly, kyphoscoliosis, generalized osteoporosis and thromboembolisms. While homocystin in the blood plasma and the urine could be proved only in the patient, the concentration of plasma methionine was much increased also in the clinically completely inconspicuous sister.