BACKGROUND: Little is known about the relationship between vertical ground reaction force (vGRF) and muscle strength while climbing stairs after total hip arthroplasty (THA). METHODS: We investigated the relationship between muscle strength and vGRF during stair climbing in 21 women with THA. Differences in the time required to climb stairs and side-to-side differences in muscle strength and vGRF during climbing were examined at 6 and 12 months. The relationship between climbing time and vGRF variables and that between vGRF and muscle strength were analyzed using Spearman's rank correlation coefficient at 6 and 12 months. First peak force (Fz1) was defined as the peak vGRF at heel strike normalized by body weight. FINDINGS: Climbing time was related to the time to Fz1 at 6 and 12 months post-THA (r = 0.50-0.64). At 6 months, time to Fz1 was related to the strength of muscles involved in hip extension in the operated limb (r = -0.46). At 12 months, time to Fz1 was related to the strength of muscles responsible for knee extension in the operated limb (r = -0.45). INTERPRETATION: At 6 months, climbing stairs depended on muscle strength in the hip extensors. Conversely, at 12 months, the functional recovery of knee extensors was important for transferring weight upward while climbing stairs. Therefore, to improve the climbing function of patients with THA, it is important to not only focus on the recovery of muscle strength in the hip extensors but also improve muscle strength in the knee extensors.