Methenamine salts are often used for the prevention of urinary tract infection (UTI). To assess the effectiveness of methenamine hippurate in preventing UTI. Published and unpublished randomised controlled trials were identified from the Cochrane Controlled Trials Register, MEDLINE, EMBASE, CINAHL, Current Contents, reference lists of review articles and retrieved trials. The manufacturers' of methenamine salts were contacted for unpublished studies and contact was made with known investigators in the area. Randomised and quasi-randomised trials of methenamine hippurate used for the prevention of UTIs in all population groups were eligible for inclusion. A comparison with a control (no treatment) group was a prerequisite to selection. Two reviewers (BL and TB) performed independent assessment and data extraction using a standardised format. Discrepancies, methodological and interpretative issues were discussed with JS or JC. An exploration of heterogeneity as well as a detailed description of results grouped by population was conducted. Eleven studies met the inclusion criteria. All trials were included in a descriptive analysis. Seven trials were included in meta-analyses. Four trials (199 patients) studied symptomatic bacteriuria and six trials (341 patients) studied bacteriuria as an outcome measure. Overall, trial quality was poor. The direction of six of the pooled trials was towards a favourable treatment effect from methenamine hippurate. Interpretation of the pooled estimates was not done in view of underlying heterogeneity. The study by Pettersson 1989 explained some, but not all, of the underlying heterogeneity. This study differed from all others by including patients with known upper renal tract abnormalities. Adverse reactions were mentioned by 10 studies. The rate of adverse events was low. There is not enough evidence to conclusively support the use of methenamine hippurate for urinary prophylaxis. An exploration of heterogeneity raises the (hypothesis generating) possibility that methenamine hippurate may have some efficacy in patients without but not in patients with known upper renal tract abnormality (with asymptomatic bacteriuria as the outcome measure). Due to the small sample size and methodological problems within the studies involved, interpretation of these data should be done cautiously. The rate of adverse events reported by the trials was low, which suggests that current usage is unlikely to be causing significant harm.