BACKGROUND: Pneumonia poses a significant burden to the U.S. health-care system. However, there are few data focusing on severe pneumonia, particularly cases of pneumonia associated with specialized care in intensive care units (ICU). METHODS: We used administrative and electronic medical record data from six integrated health care systems to estimate rates of pneumonia hospitalizations with ICU admissions among adults during 2006 through 2010. Pneumonia hospitalization was defined as either a primary discharge diagnosis of pneumonia or a primary discharge diagnosis of sepsis or respiratory failure with a secondary diagnosis of pneumonia in administrative data. ICU admissions were collected from internal electronic medical records from each system. Comorbidities were identified by ICD-9-CM codes coded during the current pneumonia hospitalization, as well as during medical visits that occurred during the year prior to the date of admission. RESULTS: We identified 119,537 adult hospitalizations meeting our definition for pneumonia. Approximately 19% of adult pneumonia hospitalizations had an ICU admission. The rate of pneumonia hospitalizations requiring ICU admission during the study period was 76 per 100,000 population/year; rates increased for each age-group with the highest rates among adults aged >/=85 years. Having a co-morbidity approximately doubled the risk of ICU admission in all age-groups. CONCLUSIONS: Our study indicates a significant burden of pneumonia hospitalizations with an ICU admission among adults in our cohort during 2006 through 2010, especially older age-groups and persons with underlying medical conditions. These findings reinforce current strategies aimed to prevent pneumonia among adults.