OBJECTIVE: To compare the 1-year costs for patients treated for acute and chronic ambulatory low back pain by medical physicians and chiropractors. STUDY DESIGN: Prospective, practice-based observational study undertaken in 13 general medical practices and 51 chiropractic community-based clinics. PATIENTS AND METHODS: Of 2872 study patients, 2263 had complete 1-year records of services. Service data, collected from billing records, chart audits, and provider questionnaires, were assigned relative value units that were converted into 1995 dollar costs. Prescription drug costs for medical patients were included. Patient data on health status, pain and disability, and socioeconomic characteristics were obtained from self-administered questionnaires. RESULTS: The direct office costs of treating both chiropractic and medical patients over a 1-year period were relatively small. Forty-three percent of chiropractic patients and 57% of medical patients incurred costs of less than $100. However, the mean costs associated with chiropractic patients ($214) were significantly higher than those for medical patients ($123), especially when compared with medical patients who were not referred for further treatment or evaluation ($103). Chiropractic patients had somewhat lower baseline levels of pain and disability than nonreferred medical patients, but the 2 groups were relatively similar on most patient characteristics. There also were no statistically significant differences in the improvements in pain and disability between these 2 groups of patients. CONCLUSION: The results of this study indicate that patients treated in chiropractic clinics incur higher costs over a 1-year period, but have about the same degree of relief as nonreferred patients treated in medical clinics.