We compared measures of treatment effectiveness when inhaled corticosteroids (ICSs) or leukotriene modifiers (LMs) were used as controller monotherapy for asthma. Asthma patients aged 6-55 years initiating ICS or LM monotherapy between July 1998 and June 1999 (index prescription) were identified using a managed care claims database. Asthma-related hospitalizations, emergency department (ED) visits, and use of short-acting beta-agonists and oral corticosteroids (OCSs) were assessed as proxies for treatment effectiveness. Propensity score was used to adjust for baseline differences between treatment cohorts. The change in the annual rate of claims from the preindex to postindex period for each measure was compared across treatment groups. Logistic regression models of the postindex composite events (hospitalization and/or ED) and OCS use were estimated. Nine hundred sixty patients were initiated on LMs (n = 153) and ICSs (n = 807). The mean annual rate of claims for OCSs increased in the ICS group (0.2) but was unchanged in the LM group (adjusted mean difference in change, 0.2; 95% CI, 0.05-0.4; p = 0.01). The mean annual rate of claims for short-acting beta-agonists increased in both the ICS and LM groups by 1.1 and 0.5, respectively (adjusted mean difference in change, 0.6; 95% CI, -0.06. 1.1; p = 0.08). Similar changes in annualized rates of claims for hospitalizations and ED visits were observed between treatment groups. In logistic regression models, greater odds of postindex OCS use was observed among the ICS group (odds ratio for ICS versus LM = 1.7; 95% CI, 1.04-2.8; p = 0.04). No association between treatment groups and postindex hospitalization and/or ED events was observed. In this managed care population, patients treated with ICSs or LMs had similar measures of treatment effectiveness, as measured by asthma-related health care resource use.