Objective: The purpose of this study was to identify short-term and long-term determinants of costs and pain improvement for care of low back pain (LBP) provided by medical doctors (MDs) and chiropractors (DCs). Methods: Determinants of office-based costs and pain improvement were modeled for 2872 patients with primary complaint of acute or chronic LBP of mechanical origin enrolled from practices of 111 MDs and 60 DCs using multiple regression analysis. The independent variables were baseline pain intensity (10 cm visual analog scale), chronicity (current episode > or <7 weeks), referred pain above/below the knee, history of LBP, physical health, depression screen, comorbidity, and stress index; age, sex, married, and smoker; pay variables including out-of-pocket, health insurance, auto insurance, Workers' Compensation, and Oregon Health Plan/Medicaid; and a choice of provider indicator based on relative confidence in DC and MD care. Results: Determinants of increased office-based costs for MD care were Workers' Compensation, pain below the knee, and chronic LBP with comorbidity. Predictors of increased cost for DC care were Workers' Compensation, auto and health insurance, LBP chronicity, and baseline pain. Predictors of decreased DC cost were Medicaid and better physical health. Pain improvement was predicted consistently across groups by baseline pain, pain radiating below the knee, physical health, LBP chronicity, and chronicity by baseline pain interaction. There was also a large chronicity by comorbidity interaction at 12 months for both provider types. Conclusions: Cost predictors were driven by insurance type and pain improvement was driven by LBP complaint characteristics.