PURPOSE: The goal of this study was to identify major cost categories of direct medical costs among patients with musculoskeletal conditions undergoing outpatient rehabilitation. MATERIALS AND METHODS: Minor cost categories were discriminated from major cost categories by ranking the percentage of direct medical costs represented by each category using a cut-off of 5%. Means and 95% confidence intervals of direct medical costs were estimated using 1000 bootstrap samples. RESULTS: Major cost categories for our sample including osteoarthritis, osteoporosis, back pain and fibromyalgia patients were outpatient physician visits, non physician services, medication and inpatient treatment. Since the 95% confidence intervals of estimated means were overlapping, no statistically significant difference can be assumed between the mean direct medical costs estimated solely with major categories and the one estimated with all available categories. DISCUSSION: The development of a standard brief core set of major direct medical cost categories for musculoskeletal conditions is important regarding the standardization of comparative economic evaluations. The major cost categories we identified can serve as candidate categories and make a valuable contribution to the establishment of a standard brief core set of major cost categories for musculoskeletal conditions which should be included in the alongside economic evaluation of rehabilitative interventions. CONCLUSION: In summary, we have identified a) medication, b) physician visits, c) non physician services use and d) inpatient treatment as major direct medical cost categories for our sample including osteoarthritis, osteoporosis, back pain and fibromyalgia patients.