Side-lying hip abductor strength tests are commonly used to evaluate muscle strength. In a "break" test, the tester applies sufficient force to lower the limb to the table while the patient resists. The peak force is postulated to occur while the leg is lowering, thus representing the participant's eccentric muscle strength. However, it is unclear whether peak force occurs before or after the leg begins to lower. To determine intrarater reliability and construct validity of a hip abductor eccentric strength test. Intrarater reliability and construct validity study. Twenty healthy adults (26 6 y; 1.66 0.06 m; 62.2 8.0 kg) made 2 visits to the laboratory at least 1 week apart. During the hip abductor eccentric strength test, a handheld dynamometer recorded peak force and time to peak force, and limb position was recorded via a motion capture system. Intrarater reliability was determined using intraclass correlation, SEM, and minimal detectable difference. Construct validity was assessed by determining if peak force occurred after the start of the lowering phase using a 1-sample t test. The hip abductor eccentric strength test had substantial intrarater reliability (intraclass correlation  = .88; 95% confidence interval, .65-.95), SEM of 0.9 %BWh, and a minimal detectable difference of 2.5 %BWh. Construct validity was established as peak force occurred 2.1 (0.6) seconds (range: 0.7-3.7 s) after the start of the lowering phase of the test (P ≤ .001). The hip abductor eccentric strength test is a valid and reliable measure of eccentric muscle strength. This test may be used clinically to assess changes in eccentric muscle strength over time.