In order to investigate the status of osteoporosis associated with rheumatoid arthritis, a histomorphometric study was performed on the specimens taken from the iliac bone of the patients who underwent total knee arthroplasty. All of them had received double tetracycline labeling preoperatively. Twenty consecutive patients admitted to Nakaizu Spa Hospital for the surgery from February to July, 1989 were subjected for the study. All the patients were women with the age ranging from 38 to 68 years (aver. 56.8 years). Seventeen out of the 20 patients were of class 3 or more according to Steinbrocker's staging. There were MUD (multilating disease), 14 MES (more erosive subset) and no LES (least erosive subset) patients according to Ochi's classification. Basically the Frost's method was used for the morphometric examination, and Parfitt's histomorphometric parameters were used for the results evaluation. The results demonstrated that bone volume and trabecular thickness were decreased whereas bone absorption and bone formation rate were increased. Histological findings showed resemblance with that of hyperparathyroidism in view of thin trabecular pattern. The osteoporosis in the present rheumatoid patients were considered to be due to abnormal bone remodeling, secondary to high turn-over of the bone. The bone in the more advanced type (MUD) patients showed much increased bone turn-over and osteoporosis than that of less advanced ones (MES). Hence force, the degree of osteoporosis was in positive relationship with the severity of rheumatoid arthritis. The postoperative clinical results were assessed by the Japanese Orthopaedic Association (JOA) Rheumatoid Knee Scoring System. Sixteen out of the 20 patients survived at the time of the follow-up (follow-up period: 4 years and 4 months to 6 years and 8 months). Among them, 11 patients were directly examined and the remaining five patients answered a questionnaire. The JOA score showed no correlation with bone volume or bone formation rate, but significant negative correlation with bone eroded surface and osteoclast surface in the morphometric examination.