Objective: To increase the knowledge of the long-term effects of artificial hip and knee joint implants. Methods: The study groups consisted of 24,636 patients with osteoarthritis who underwent hip implant surgery and 5,221 who received knee implants during 1977-89. The post-implant rate of hospitalization for connective tissue disease (CTD) was compared with the rate in the general population of Denmark and with that among osteoarthritis patients without implant surgery. Results: The rates of hospitalization for CTD were higher than the background level among both hip and knee implant patients with osteoarthritis, whereas the comparison with non-implanted osteoarthritis patients revealed that the hospitalization rate for CTD was reduced after hip implant surgery, but increased after knee implant surgery. Conclusion: Since the materials used in hip and knee implants in Denmark are not substantially different, these results are unlikely to reflect an implant effect but rather the selection criteria of referral for implant surgery.