HISTORY: Pronounced osteoporosis was discovered in a 44-year-old man presenting with an herniated intervertebral disk. He reported severe myopia, incomplete dislocation of the lenses and retinal detachment. He did not know of any thrombembolic events in the past. On physical examination Marfan-like appearance and a funnel chest were noted. Because of these findings Marfan syndrome was suspected. INVESTIGATIONS: Considering the findings of the x-rays, homocystinuria was suspected as a cause of osteoporosis, despite apparently normal cognitive functions. This diagnosis was confirmed by greatly increased values of serum homocysteine and a positive test for urinary homocystine. Since methionine and homocystine were both elevated, the diagnosis of cystathionin-beta-synthase deficiency was established. DIAGNOSIS, TREATMENT AND COURSE: After taking folate and vitamin B6, the homocysteine level decreased moderately. Betaine and subsequently vitamin B12 were added. Homocysteine values declined markedly on this therapeutic regimen. Two years later the patient was admitted again because of atypical angina. Coronary heart disease could be excluded by coronary angiography. CONCLUSION: Diagnosis of premature osteoporosis should prompt consideration of homocystinuria even in adults. Premature arteriosclerosis, thrombembolic diseases, dislocation of the lens and retinal detachment may give further clues.