Eighty inpatients with established osteoarthrosis received either 1.5 gm of glucosamine sulfate or placebo daily, in three divided oral doses, for 30 days. Articular pain, joint tenderness and swelling, and restriction of active and passive movements were scored at one week intervals, as were possible side reactions. Hematologic analysis, urine analysis, and occult blood in feces were recorded before and after treatment. Samples of articular cartilage from two patients of each group and from one healthy subject were submitted to scanning electron microscopy after the end of treatment. All symptoms decreased in both groups. The patients treated with glucosamine sulfate experienced a reduction in overall symptoms that was almost twice as large (73% vs. 41%) and twice as fast (time to reduce symptoms by 50%: 20 days vs. 36 days) as those who had placebo. the improvement of autonomous mobility was relatively less, compared to improvement in the other symptoms, for patients with placebo; with glucosamine slfate, on the contrary, the improvement was as great and as fast as that of the other symptoms. Thus a direct action of glucosamine sulfate on the cartilage is hypothesized. The hypothesis is supported by the findings of electron microscopy. The patients who had placebo showed a typical picture of established osteoarthrosis. Those who had glucosamine sulfate showed a picture more similar to healthy cartilage. It is concluded that glucosamine sulfate tends to rebuild the damaged cartilage, thus restoring articular function in most chronic arthrosic patients.