A study based on the behaviour of hydroxyproline in 64 cases, 16 normal individuals, 10 with rheumatoid arthritis, 5 with systemic lupus erythematosus, 30 with osteoporois, 2 with vitamin D resistant rickets and 1 with osteomalacia, is presented, hydroxyproline is an amino acid derived from proline and is actually considered of great value in the study of collagen metabolism. The results obtained in each group of disease are presented and compared with those obtained in the normal individuals. We think that these late are very useful to establish in our medium, the average of normal free serum hydroxyproline and of total and free urine hydroxyproline, since we did not find related data in the consulted ational literature.