Background: Existing comorbidity indices were not developed for adolescent and young adults (AYA) 15 to 39 years of age. The aim of this study was to assess impact of comorbidities on health care service needs and health status among AYA cancer survivors using the newly developed AYA HOPE comorbidity index in comparison with the existing indices. Methods: Data on comorbid conditions were obtained from medical records and service needs and health status were from a survey of AYA cancer survivors. Prevalence of comorbidities was based on the AYA HOPE index. Charlson and NCI indices were compared. Multivariable logistic regression was used. Results: Of the 485 patients, 14.6% had ≥2 comorbidities based on the AYA HOPE Index. Prevalence of mental illness and obesity/overweight, which were not included in existing indices, were 8.2% and 5.8%, respectively. Prevalence of cardiovascular, endocrine, gastrointestinal, and neurologic conditions were higher with the AYA HOPE Index than the other two indices. Forty percent of AYA patients reported service needs, particularly for mental health services (25.2%) and support groups (17.7%). Having ≥2 comorbidities on the AYA index was associated with higher mental health service needs [OR, 2.05; 95% confidence interval (CI), 1.10-3.82] adjusting for demographic and clinical factors. Comorbidities were associated with fair/poor selfreported health status. Conclusion: The AYA HOPE Index is a more comprehensive comorbidity index for AYA cancer patients than existing indices, and the number of comorbidities is associated with service needs and health status. Impact: The AYA HOPE index could identify patients' additional service needs early in therapy. Cancer Epidemiol Biomarkers Prev; 24(12); 1844-9.