CRD Summary: The authors concluded that azathioprine showed a modest efficacy in the treatment of ulcerative colitis, but the quality of evidence was not high. This was a generally well-conducted study, but the lack of available high-quality studies and the small number of participants meant that the reliability of the authors' conclusions was unclear.

CRD Commentary: The review addressed a clear question. Inclusion criteria were well-defined. Several relevant databases were searched. Attempts were made to identify unpublished data and publication bias was assessed with no evidence of bias was found, but the assessment was of limited value due to the small number of studies. No language restrictions were applied to the search, reducing the possibilty of language bias. Three of the five studies included in the meta-analysis were conducted by the same group of researchers. Appropriate steps were taken in the study selection, data extraction and validity assessment processes to minimise reviewer error and bias. A suitable tool was used to measure methodological quality and was used to inform the meta-analysis. Given the presence of statistical and clinical heterogeneity, a narrative synthesis may have been more appropriate. This was a generally well-conducted study. However, the lack of available high quality studies, differences between the studies and the small number of participants meant that the reliability of the authors' conclusions was unclear.