This investigation assessed strength of the hip extensors and flexors when assistive devices and weight bearing are changing after total hip arthroplasty (THA). Eleven individuals (6 men, 5 women; mean age 74.45 +/- 4.88 years) with unilateral THA were evaluated isokinetically at 60 degrees x sec(-1) before surgery on the involved and uninvolved limbs. Each subject's involved limb was tested 60 days after surgery. Comparisons were made between involved and uninvolved limbs and between the involved limb before surgery and 60 days after surgery for both the hip extensors and flexors. Hip extensor and flexor strength before surgery on the involved side was 39% and 29% lower, respectively, compared with the uninvolved side. Sixty days after surgery, strength of the hip extensors and flexors improved 50% and 27%, respectively, compared with before surgery. Over the 60-day interval, the responsiveness of isokinetic testing was high for both muscle groups (range, 0.74-1.51). It would seem appropriate that intensive rehabilitation continue through at least the 60-day period and that isokinetic testing is an effective tool to monitor hip strength before and after surgery.