OBJECTIVE: We aimed to develop and validate administrative data-based comorbidity indices for a range of cancer types that included all relevant concomitant conditions. STUDY DESIGN AND SETTINGS: Patients diagnosed with colorectal, breast, gynecological, upper gastrointestinal, or urological cancers identified from the National Cancer Registry between July 1, 2006 and June 30, 2008 for the development cohort (n=14,096) and July 1, 2008 to December 31, 2009 for the validation cohort (n=11,014) were identified. A total of 50 conditions were identified using hospital discharge data before cancer diagnosis. Five site-specific indices and a combined site index were developed, with conditions weighted according to their log hazard ratios from age- and stage-adjusted Cox regression models with noncancer death as the outcome. We compared the performance of these indices (the C3 indices) with the Charlson and National Cancer Institute (NCI) comorbidity indices. RESULTS: The correlation between the Charlson and C3 index scores ranged between 0.61 and 0.78. The C3 index outperformed the Charlson and NCI indices for all sites combined, colorectal, and upper gastrointestinal cancer, performing similarly for urological, breast, and gynecological cancers. CONCLUSION: The C3 indices provide a valid alternative to measuring comorbidity in cancer populations, in some cases providing a modest improvement over other indices.