e16626 Background: Cancer patients in late adolescence and young adulthood may be referred for treatment to either pediatric or medical oncologists; research is exploring the impact of specialty and treatment regimen on outcomes. Recognizing the role of the initial referral decision by the primary care physician (PCP), we sought to determine physician and patient factors that influence referral patterns of AYA oncology patients. Methods: Surveys were mailed to 1129 PCPs: 50% pediatricians, 50% non-pediatricians with representative distribution of Family Medicine, Internal Medicine, General Practice, Emergency Medicine, Gynecology in North Carolina (NC) and Washington (WA). PCPs provided demographics, then reviewed hypothetical patient vignettes that independently varied 8 patient factors by fractional factorial design: age (13-22), gender, diagnosis, race, social support, insurance, transportation, patient preference for site of care, and denoted to whom (pediatric or non-pediatric oncologist) they would refer the patient. Results: Response rate was 33.2% in NC and 38.6% in WA, totaling 406 completed surveys. By univariate and multivariate analysis, the only significant PCP characteristic was specialty; referral to a pediatric oncologist was made in 60% of vignettes by pediatric PCPs and 36.5% of non-pediatrician PCPs (p-value < 0.0001). Regardless of PCP specialty, referral patterns shifted with patient age: 89% of 13 year olds (yo), 74% of 16 yo, 25% of 19 yo and 9% of 22 yo were referred to pediatric oncologists. Compared to germ cell tumors, acute lymphoblastic leukemia (OR = 4.29; 95% CI: 2.77 - 6.64), Hodgkin’s lymphoma (OR = 3.88; 95% CI: 2.57 - 5.84), and Ewing’s sarcoma (OR = 4.77; 95% CI; 3.10 - 7.35) were more likely to be referred to a pediatric provider. Those lacking private insurance were more likely referred to pediatric oncologists (OR 1.42; 95% CI 1.09-1.87). Conclusions: PCP specialty, patient age, type of cancer and insurance status were significant drivers of referral patterns for AYA cancer patients. Referrals to pediatric oncology drop dramatically over age 18. Because site of oncologic care can impact outcomes, awareness and education efforts regarding AYA cancer should include PCPs.