This article summarizes a study showing the major factors that affect referral rates of Oklahoma City Area Indian Health Service primary-care professionals. The article also summarizes a referral rate prediction model. The multiple linear regression statistical analysis, employed in the study, produced three factors that largely determined the referral rates of physicians practicing in tertiary-care and secondary-care facilities: hospitalized morbidities per 1,000 population, the total medical-care professionals associated with a facility, and the number of specialists associated with a facility. The referral rates of physicians practicing in primary-care facilities were associated with morbidities per 1,000 population, median age of primary-care professionals, and distance to the closest medical referral center. The findings of this article take on new meaning and importance with the advent of diagnostic related group (DRG) reimbursement legislation. If administrators understand more about physician referrals, they will better understand one significant aspect of the cost of DRGs in their facilities.