Editor,—We were interested to read the study by Evans et al (Gut1997;40:619–22) as we have recently reported a prospective study of colitis drawing similar conclusions.¹Our study, however, showed a much stronger association. Forty five of 62 (72.4%) new cases of colitis were taking non-steroidal anti-inflammatory drugs (NSAIDs) or salicylates compared with 38 (7%) of 513 of a control population sample (odds ratio 33.1, 95% confidence interval 17.31 to 63). This difference between the two papers is almost certainly owing to the fact that our group of colitics comprised a much broader spectrum of disease than those reported by Evans et al, who were clearly only studying patients with colitis in relapse.

In addition, we enquired about the usage of over-the-counter NSAIDs and salicylates which was not possible in the study design of Evanset al. We would maintain that exposure misclassification could easily have occurred if over-the-counter usage of these compounds was not considered.

The incident cases of Evans et al were de novo cases of …