Objective: To make a preliminary appraisal of the types of unpleasant reactions reported by patients after spinal manipulation and to estimate their frequency. Study design: Standardized interview data collected in a longitudinal survey. Method: Ten chiropractors collected data on all unpleasant reactions that were reported after a maximum of six visits by ten consecutive patients per chiropractor, with the use of a questionnaire that contained mainly closed-ended questions. Results: Following 368 treatments in 95 patients, some type of side-effect was reported after 1/3 of treatments. Local or radiating symptoms were most commonly reported (23%). No alarming events were reported. Ninety percent of all reactions were graded by the patients as moderate or slight. They commenced on the day of therapy in 87% of cases, and had disappeared within 24 hours in 83%. Conclusion: Results from this preliminary study indicate that reactions to spinal manipulation may be relatively common but benign in nature and of short duration. No estimate could be made of rare or serious side-effects.