465 patients were randomized to assess whether the timing of aquatic therapy influenced clinical outcomes after total knee arthroplasty (TKA) or total hip arthroplasty (THA). 280 THA patients and 185 TKA patients received aquatic therapy (pool exercises aimed at training of proprioception, coordination, and strengthening) on the 6th or 14th postoperative day after THA or TKA. Results at 24 months revealed that the self-reported physical function was superior in the early aquatic therapy group for TKA patients, while the late aquatic therapy showed superior results for THA patients. However, neither of these findings reached clinical significance.