In so-called primary osteoarthritic knees, there may be secondary osteoarthritis due to the instability from chronic cruciate ligament and/or meniscal tear. The purpose of this study is to compare the findings of bone single-photon emission computed tomography(SPECT) and those of simple radiography in osteoarthritic knees which we consider as primary on the status of anterior cruciate ligament(ACL). We reviewed the preoperative bone SPECT and simple radiography of forty-three osteoarthritic knees in 23 patients who underwent total knee arthroplasty between 1995 and l996. We divided the cases into two groups on the status of ACL: thirty intact ACLs(Group I), thirteen insufficient ACLs(Group II). Meniscal tear and/or ACL insufficiency were found in 38 of 43 knees, As regards with presence of osteophytes and sclerotic changes on simple radiograph, there was no significant differences between group I and II. Joint space narrowing was more prominent and diffuse in group I(medial 29, lateral 24) than group II(medial 7, lateral 6). Bone SPECT showed diffuse uptake in group I(medial 30, lateral 23), but less uptake on the lateral compartment(medial 13, lateral 3) in group II. In ACL-intact osteoarthritic knees, joint space narrowing on simple radiograph and hot uptake on bone SPECT were more prominent and diffuse than in ACL-insufficient knees. Clinical relevance is still uncertain and further investigation is needed.