OBJECTIVES: The transition from ICD-9-CM to ICD-10-CM on 10/1/15 raises potential concerns due to differences in diagnosis code specificity across certain conditions. This may lead to inconsistent patient counts and comorbidity indices during the transition to ICD-10-CM. In this study, patient counts for six common conditions and mean Quan-Charlson comorbidity scores in the two months prior to and one month after the transition date were compared in a large, US managed care database. METHODS: This retrospective analysis used medical claims data for commercial members enrolled in a large, US managed care plan between 8/1/15-10/31/15. Patients with at least 1 medical claim for one of 6 conditions were retained: type 2 diabetes mellitus (T2DM); depression; lung cancer; rheumatoid arthritis; atherosclerotic cardiovascular disease (ASCVD); and chronic obstructive pulmonary disease (COPD). ICD-9-CM and ICD-10-CM code lists were constructed by an M.D. medical director who is a coding specialist. Monthly patient counts for each condition were determined. The monthly mean Quan-Charlson comorbidity scores for all three months were also calculated. RESULTS: Patient counts for T2DM (297,382; 300,448; and 288,974), lung cancer (9,639; 9,606; and 9,293) and RA (30,755; 31,550; and 30,360) were similar across the transition period. However, patient counts for depression dropped slightly (298,254; 298,473; and 266,058) while counts for COPD (30,178; 32,828; and 103,312) and ASCVD increased dramatically (86,619; 87,285; and 124,400). The mean Quan-Charlson score was consistent across the three month period. CONCLUSIONS: In this large, US managed care database analysis, the transition from ICD-9-CM codes to ICD-10 codes resulted in consistent patient counts across 3 of 6 conditions. These findings indicate the importance of a careful review of the code lists during this transition period as well as the need for investigation of the reasons for the inconsistencies.