Background Ultrasound is used in the treatment of a wide variety of musculoskeletal disorders, which include acute ankle sprains. This is an update of a Cochrane review first published in 1999, and previously updated in 2004. Objectives To evaluate the effects of ultrasound therapy in the treatment of acute ankle sprains. Search strategy We searched the Cochrane Bone, Joint and Muscle Trauma Group Specialised Register (September 2010), the Cochrane Central Register of Controlled Trials (The Cochrane Library 2010, Issue 3), MEDLINE (1966 to September 2010), EMBASE (1983 to September 2010), CINAHL (1982 to 2004), and PEDro - the Physiotherapy Evidence Database (accessed 01/06/09). We also searched the Cochrane Rehabilitation and Related Therapies Field database, reference lists of articles, and contacted colleagues. The WHO International Clinical Trials Registry Platform was searched for ongoing trials. Selection criteria Randomised or quasi-randomised trials were included if the following conditions were met: at least one study group was treated with therapeutic ultrasound; participants had acute lateral ankle sprains; and outcome measures included general improvement, pain, swelling, functional disability, or range of motion. Data collection and analysis Two authors independently performed study selection, and assessed the risk of bias and extracted data. Risk ratios and risk differences together with 95% confidence intervals were calculated for dichotomous outcomes and mean differences together with 95% confidence intervals for continuous outcome measures. Limited pooling of data was undertaken where there was clinical homogeneity in terms of participants, treatments, outcomes, and follow-up time points. Main results Six trials were included, involving 606 participants. Five trials included comparisons of ultrasound therapy with sham ultrasound; and three trials included single comparisons of ultrasound with three other treatments. The assessment of risk of bias was hampered by poor reporting of trial methods and results. None of the five placebo-controlled trials (sham ultrasound) demonstrated statistically significant differences between true and sham ultrasound therapy for any outcome measure at one to four weeks of follow-up. The pooled risk ratio for general improvement at one week was 1.04 (random-effects model, 95% confidence interval 0.92 to 1.17) for active versus sham ultrasound. The differences between intervention groups were generally small, between zero and six per cent, for most dichotomous outcomes. Authors' conclusions The evidence from the five small placebo-controlled trials included in this review does not support the use of ultrasound in the treatment of acute ankle sprains. The potential treatment effects of ultrasound appear to be generally small and of probably of limited clinical importance, especially in the context of the usually short-term recovery period for these injuries. However, the available evidence is insufficient to rule out the possibility that there is an optimal dosage schedule for ultrasound therapy that may be of benefit.