Compliance with prescribed asthma medication is commonly estimated from tablet counts for oral medications and canister weights for inhaled medications. Recently, electronic medication monitoring devices, developed to evaluate numerical compliance as well as drug use patterns, were used to assess compliance with inhaled steroids and beta2-agonists. This was the first study to electronically assess compliance with an oral asthma medication. Fifty-seven asthmatic patients, stable on inhaled beta2-agonists only with a mean FEV1 of 77% predicted (+/- 13%, SD) began 12 weeks of treatment with zafirlukast 20 mg twice daily. The monitoring device, an electronic TrackCap, recorded the date and time on each occasion that patients removed and replaced their medication bottle caps. Patients were told that compliance would be assessed as part of the study, but patients were not told about the specifics of the TrackCap. Compliance was defined: 1. as the number of TrackCap events per number of prescribed tablets; and 2. as the difference between number of tablets dispensed and number returned per number prescribed. Adherence was defined as the number of days with two TrackCap events at least 8 h apart per the total number of days' dosing. Forty-seven patients completed the study with a median compliance of 89% (mean. 80%) and a median adherence of 71% (mean, 64%) as measured by TrackCap events. Compliance as estimated from return-tablet count was slightly higher (median, 92%). High rates of compliance were maintained throughout the trial. These results show that compliance with and adherence to a treatment of an oral, twice-daily, maintenance asthma medication, such as zafirlukast, is high.