Serum C-reactive protein (CRP) levels and erythrocyte sedimentation rates (ESR) were measured serially in 50 osteoarthritic patients treated with uncomplicated primary total hip arthroplasties (THAs) and also in 28 patients with painful hip for different periods after THA. The follow-up period was one year. In uncomplicated primary THA, the ESR was slightly elevated before surgery and was variable after surgery; some patients continued to have abnormally high ESRs one year after surgery. However, CRP was normal before surgery and after surgery showed a distinctive pattern that rapidly normalized within three weeks. In patients with a painful hip after THA, ESR was elevated in those with septic loosening and variable in those with mechanical loosening; however, the difference was not significant. ESR after successful revision THA was also variable. CRP was high in patients with septic loosening and normal in those with mechanical loosening. Normalization of CRP was rapid after control of infection or within three weeks of successful revision THA. Uneventful recovery after THA is indicated by normalization of CRP within three weeks, regardless of ESR. Measurement of both ESR and CRP is helpful in differentiating septic and mechanical loosening, detecting complications, and monitoring the effects of treatment, but CRP is more informative and sensitive than ESR for these purposes.