BACKGROUND: Acute lateral ankle ligament injuries (ankle sprains) are common problems in acute medical care. The treatment variation observed for the acutely injured lateral ankle ligament complex suggests a lack of evidence-based management strategies for this problem. OBJECTIVES: The objective of this review was to assess the effectiveness of methods of immobilisation for acute lateral ankle ligament injuries and to compare immobilisation with functional treatment methods. SEARCH STRATEGY: We searched the Cochrane Musculoskeletal Injuries Group specialised register (December 2001); the Cochrane Controlled Trials Register (The Cochrane Library, Issue 4, 2001), MEDLINE (1966-May 2000), EMBASE (1988-May 2000), reference lists of articles, and contacted organisations and researchers in the field. SELECTION CRITERIA: Randomised and quasi-randomised controlled trials comparing either different types of immobilisation or immobilisation versus functional treatments for injuries to the lateral ligament complex of the ankle in adults were included. Trials which investigated the treatment of chronic instability or post-surgical treatment were excluded. DATA COLLECTION AND ANALYSIS: Data were independently extracted by two authors. Where appropriate, results of comparable studies were pooled using fixed effects models. Individual and pooled statistics were reported as relative risks with 95% confidence intervals for dichotomous outcomes and weighted (WMD) or standardised (SMD) mean differences and 95% confidence intervals for continuous outcome measures. Heterogeneity between trials was tested using a standard chi-squared test. MAIN RESULTS: Twenty-one trials involving 2184 participants were included. The mean validity score of the included trials increased from 9.1 (SD 3.0) to 10 (SD 2.9) after retrieving further information (maximum 18 points). Statistically significant differences in favour of functional treatment when compared with immobilisation were found for seven outcome measures: more patients returned to sport in the long term (relative risk (RR) 1.86, 95% confidence interval (CI) 1.22 to 2.86); the time taken to return to sport was shorter (WMD 4.88 (days), 95% CI 1.50 to 8.25); more patients had returned to work at short term follow-up (RR 5.75, 95% CI 1.01 to 32.71); the time taken to return to work was shorter (WMD 8.23 days, 95% CI 6.31 to 10.16); fewer patients suffered from persistent swelling at short term follow-up (RR 1.74, 95% CI 1.17 to 2.59); fewer patients suffered from objective instability as tested by stress X-ray (WMD 2.60, 95% CI 1.24 to 3.96); and patients treated functionally were more satisfied with their treatment (RR 1.83, 95% CI 1.09 to 3.07). A separate analysis of trials that scored 50 per cent or more in quality assessment found a similar result for time to return to work only (WMD (days) 12.89, 95% CI 7.10 to 18.67). No significant differences between varying types of immobilisation, immobilisation and physiotherapy or no treatment were found, apart from one trial where patients returned to work sooner after treatment with a soft cast. In all analyses performed, no results were significantly in favour of immobilisation. REVIEWER'S CONCLUSIONS: Functional treatment appears to be the favourable strategy for treating acute ankle sprains when compared with immobilisation. However, these results should be interpreted with caution, as most of the differences are not significant after exclusion of the low quality trials. Many trials were poorly reported and there was variety amongst the functional treatments evaluated.