Every article sent by an author to a reputed scientific journal undergoes a rigorous editorial evaluation. The editor has the final responsibility of accepting or rejecting manuscripts and thus can confer authority and validity on the author's research and help to disseminate new knowledge. In this task, editors make use of a panel of expert peer reviewers in the field who examine the manuscript and make recommendations. Some aspects of the peer review process have been investigated by medical scientists and by linguists but to the best of our knowledge, there have been no studies conducted on peer reviewer comments of medical articles written in English by Italian researchers. The present study aimed to establish the most frequent types of comments made by peer reviewers to identify the most frequent linguistic problems that Italian researchers encounter in this process. A collection of clinical research manuscripts submitted by Italian medical researchers to reputable English language journals were analysed together with the comments by editors and reviewers. The most frequent comments and criticisms were mainly in the area of scientific and methodological content, followed by lexical and grammatical errors, clarity and verbosity or repetition. An awareness of the features which might affect the acceptance or rejection of manuscripts may help novice writers and furnish training materials to aid researchers in writing publications in English. (Contains 1 figure and 3 tables.)