Total hydroxyproline excretion in 24-hour urine was determined in 20 cases of systemic lupus erythematosus (SLE) before and during treatment. Additionally the excretion of hydroxyproline was determined in 14 patients receiving steroids for various other diseases (endocarditis, thrombocytopenia, acute leukemia, apofocal polyarthritis, ischialgia, spastic bronchitis, pleuritis and Dressler's post-infarction syndrome). The normal hydroxyproline value was established in 40 healthy persons. The normalization of pathologically increased hydroxyproline excretion in patients with SLE during treatment with prednisone in doses from 1.0 to 1.35 mg/kg/daily was associated with remission of clinical symptoms. Large doses of prednisone in pathological processes not primarily affecting connective tissue increased the 24-hour excretion of hydroxyproline and small doses of prednisone had no effect on hydroxyprolinuria.