['Comorbidity is a frequent and often therapeutically limiting problem in older cancer patients. However, to date, there is no standard measure of the comorbidity burden available for these patients. We tested the performance of two comorbidity scales and their relationship with functional status.', 'The Cumulative Illness Rating Scale-Geriatric (CIRS-G) was compared with the Charlson scale in 203 patients who received a comprehensive geriatric assessment (CGA) in our Senior Adult Oncology Program (SAOP). Study end points were variability, reliability, correlation with Eastern Cooperative Oncology Group (ECOG) performance status (PS), Activities of Daily Living (ADL), and Instrumental Activities of Daily Living (IADL). The relative weight of comorbidity versus tumor stage in the correlations with functional status was assessed.', 'Median age was 75 years (range, 63 to 91). Sixty-four percent of patients scored 0 on the Charlson scale versus 6% on the CIRS-G. The correlation between the Charlson and CIRS-G was fair (p = 0.25 to 0.39). CIRS-G grade 3/4 had a fair correlation with ADL (p = 0.27). Otherwise, there was low or no correlation between comorbidity and functional status across the measures. Tumor stage was not correlated with functional status either. Correlation of ECOG PS with ADL (p = 0.51)c and IADL (p = 0.61) was moderate. Interrater and test-retest correlations were good or very good for both the Charlson and CIRS-G.'] Are comorbidity and functional status independent in older cancer patients?