This paper describes a careful and thorough systematic review of the effect of exercise on mood in patients with depressive symptoms. The main findings of the work are that most of the trials identified are of poor quality, and have been conducted in non- clinical populations. The main conclusion of the review is that the evidence to support clinical use of exercise in depressed patients is weak. The methods used appear to be sound, and have been well reported. The search strategy, quality filters and methods for dealing with heterogeneity are all appropriate and adequately described. The investigation by meta-regression of the sources of heterogeneity appears to me to be a particular strength and an interesting aspect of the work.

There are minor clerical errors in the results section. On p8, and in Fig 1, it is stated that of 62 relevant reports, 56 were excluded and 16 remained! Also on p8, (results, para 2) ten studies are mentioned in which exercise is compared with no treatment – on p10 and in Table 1 eleven such studies are described.

The discussion largely does justice to the findings. It may be helpful to state clearly that there is genuine doubt about the persistence of any benefits beyond the end of treatment. The broad clinical conclusions appear to be a fair reflection of the findings.

Overall this work is of a high standard, the clinical topic is important, and I would support its publication with minor revision to correct clerical errors, although (inevitably for a systematic review) the full report is well beyond the normal length of a BMJ article. (I imagine that tables and references could be put in an appendix or made available from the web). Clearly the final decision about the relative importance of the topic will remain an editorial decision.

Robert Peveler