OBJECTIVE: To examine determinates of asthma cost in a large employer population. METHODS: Data was drawn from the MedStat database. Medstat is a claims-based database with over 5 million members. Individuals were identified as asthmatics based on either a hospitalization, emergency room (ER) or outpatient visit with a primary diagnosis of asthma. Final sample size was 107,432. RESULTS: Overall mean expenditures for inpatient, outpatient and emergency room care were $417, the majority attributable to outpatient care ($270). Both ER visits (2.6% of the sample) and inpatient stays (2.5%) were relatively infrequent. But, among ER users, mean ER expenditures ($438) were greater than total mean expenditures for the typical asthmatic. Similarly, mean hospital expenditures for those using inpatient services were large ($5,316). ER visits were most likely for those aged 18-34 and became more infrequent with age. Conversely, hospital stays were least frequent for those 18-34 and otherwise showed no age trend. Although the frequency of inpatient events didn't increase with age, the cost was positively correlated with age, with the lowest mean inpatient expenditures for the 18-34 age group ($98) and the highest for those age 65 and over ($355). Outpatient costs showed a similar trend, with the highest costs for those over 65 ($355) and the lowest for the 0-18 age group ($237). Overall costs in the over 65 age group ($589) were significantly greater than the lowest cost group, ages 18-34 ($375). Mean total costs for women ($446) were higher than for men ($379), largely due to higher outpatient costs. Women more frequently used inpatient services (2.7% versus 2.4%), but were less likely to use the ER (2.5% versus 2.9%). CONCLUSIONS: Asthma is a high cost chronic illness in employer populations. Strategies to identify and manage high cost individuals may lead to cost savings.