During the period October 1975-June 1977, 70 Marmor knee arthroplasties were performed. The operative procedure provided a good opportunity to study the anatomical conditions in vivo. The causes of extension deficit and lateral instability of the knees were analysed in a prospective investigation. Thirty-seven out of 69 knees had an extension deficit exceeding 10 degrees preoperatively. At operation a bony impediment to extension was found in 32 of these 37 knees, and by removal of this obstruction the extension deficit was relieved in 31 of the knees. At radiography a bony impediment was demonstrated preoperatively as a cause of an extension deficit in 29 of the 32 knees. No false positive diagnosis was made radiographyically. Varus or valgus instability exceeding 5 degrees was observed preoperatively in 42 of the the 70 knees. Lateral stability was achieved in all knees at operation by compensating for the intra-articular loss of cartilage and bone with the Marmor module system. Macroscopically intact collateral ligaments were found in all knee joints even in those severely affected by rheumatoid arthritis. All knees except three showed lateral stability 1 year postoperatively.