CRD Summary: The authors concluded that oral theophylline improved both trough and peak forced expiratory volume and forced vital capacity in patients with stable chronic obstructive pulmonary disease. Given the methodological limitations of the review, and the unknown quality of the included trials, the authors' conclusions may not be reliable.

CRD Commentary: The review addressed a clear question. It supported by appropriate inclusion criteria, but trials may have been included that did not meet the a priori criteria. The authors restricted the search to English language publications, introducing the potential for language and publication biases, so some trials may have been missed. Validity was not assessed, making it difficult to assess the reliability of the data. As cross-over trials were included, it is important to ensure that they have been appropriately analysed so that the potential for cross-over effects has been considered. The number of reviewers involved in the systematic review process was not reported, potentially introducing reviewer bias. Details of the included trials were reported. Heterogeneity was not reported, although forest plots were presented. Given the methodological limitations of the review, and the unknown quality of the included trials, the authors' conclusions may not be reliable.