OBJECTIVES: The objective of this study is to evaluate the effect of health plan structure on health care utilization and expenditures for asthma care. Some health plans in the USA require a designated primary care physician (PCP plans) and some do not (NPCP plans). METHODS: Our data was drawn from the MEDSTAT MarketScan database. The sample had 330,685 patients with either at least one hospitalization or at least two outpatient visits for asthma during 1998-2001. Approximately 47% (154,106) of the patients had PCP plans; the remainder had NPCP health plans. We performed regression analysis to examine the effect of having a primary care physician and capitation in the health plan on expenditures for asthma, controlling for gender, age, employment status, and health status via the Charlson comorbidity index. RESULTS: We found annual per capita inpatient expenditures for asthma were about 8.4% lower for the patients in the PCP plans compared to the patients in the NPCP plans (p <0.01), with a mean length of stay 0.07 days shorter (p <0.01) and 0.02 times fewer admissions (p <0.01) per year for asthma. However, annual per capita outpatient expenditures for asthma were 9.1% higher in the PCP plans compared to the NPCP plans (p <0.01) as PCP asthma patients received 1.4 more outpatient services per year (p <0.01) and about 0.2 more outpatient visits (p <0.01) per year than NPCP asthma patients. On net, total expenditures were 3.2% lower for PCP asthma patients (p <0.01) than for NPCP asthma patients. CONCLUSIONS: For asthma care, there is a tradeoff between relatively inexpensive outpatient services and more expensive inpatient services. Results of this study suggest that patients with asthma enrolled in PCP plans used significantly more outpatient services and fewer inpatient services, resulting in lower overall spending.