STUDY DESIGN: A prospective clinic-based survey. OBJECTIVES: To study the type, frequency, and characteristics of unpleasant side effects after spinal manipulative therapy. SUMMARY OF BACKGROUND DATA: Spinal manipulative therapy is a commonly used treatment, but there is little knowledge of its side effects. METHODS: Information regarding unpleasant reactions after spinal manipulative therapy was collected after 4712 treatments on 1058 new patients by 102 Norwegian chiropractors (response rate, 70%) through structured interviews. RESULTS: At least one reaction was reported by 55% of the patients some time during the course of a maximum of six treatments. Of the reported reactions, the most common were local discomfort (53%), headache (12%), tiredness (11%), or radiating discomfort (10%). Reactions were mild or moderate in 85% of patients. Sixty-four percent of reactions appeared within 4 hours of treatment, and 74% had disappeared within 24 hours. Uncommon reactions were dizziness, nausea, hot skin, or "other" complaints, each accounting for 5% or less of reactions. It was unusual that symptoms commenced later than on the day of or the day after treatment, were of long duration (not gone at the latest on the day after onset), described as severe, or that they resulted in reduced activities of daily living. There were no reports of serious complications in this study. CONCLUSIONS: Profiles now are described of common and uncommon reactions to spinal manipulative therapy and their latency, duration, and severity.