OBJECTIVE: To (1) quantify changes in knee-extension strength and functional-performance at discharge after fast-track total hip arthroplasty (THA) and (2) investigate whether these changes correlate to changes in thigh and knee circumference (ie, swelling) or pain. DESIGN: A prospective, descriptive, hypothesis-generating study. SETTING: A special unit for fast-track hip and knee arthroplasty operations at a university hospital. PARTICIPANTS: Twenty-four patients (20 women and 4 men; ages 69 ± 6.1 years) scheduled for primary unilateral THA. METHODS: All patients were evaluated before surgery and on the day of hospital discharge. MAIN OUTCOME MEASURES: Knee-extension strength, thigh and knee joint circumference, hip pain, and functional performance (Timed Up & Go, 30-Second Chair Stand, and 10-Meter Walk tests). RESULTS: All investigated variables changed significantly from before to after surgery, except for hip pain. The average loss in knee-extension strength after surgery (32%, P = .01) did not correlate with increased thigh circumference (6%, P < .01) or knee circumference (3%, P < .01) or with reductions in functional performance: Timed Up & Go test (114%, P = .01), 30-Second Chair Stand test (36%, P = .01), and 10-Meter Walk test (50%, P < .01). Only the increase in knee circumference correlated significantly with reduced performance in the 10-Meter Walk test time (R = -0.59, P < .01), explaining 34% of the variance in the 10-Meter Walk test. No correlations between changes in hip pain and functional performance or knee-extension strength were found. Mean postoperative hospital stay was 2.1 days. CONCLUSIONS: Knee-extension strength is considerably reduced at discharge after THA, but the early strength reduction does not correlate with changes in thigh or knee circumferences. Because functional performance is also considerably reduced at discharge (unrelated to reduced knee-extension strength), other mechanisms such as fear, avoidance of movement, or decreased hip-muscle strength also may be involved.