BACKGROUND: Measurement of nebulizer medication adherence that relies on self-report of medication use is subject to recall bias and increased patient burden. Electronic monitoring of nebulizer medication use is relatively new technology and provides an objective measure of nebulizer use. OBJECTIVE: To examine levels of agreement for nebulizer use between self-report on diary cards and electronic monitor data in young inner-city children with asthma. METHODS: Of 221 enrolled children with persistent asthma, 157 (71%) provided 12 weeks of diary card and nebulizer monitor recordings that were matched by date across days and by patient. Concordance, sensitivity, specificity, and kappa coefficients were calculated between self-report and electronic data. RESULTS: The children were predominantly African American (89%) and male (66%), with a mean age of 4.6 years. Their persistent asthma was categorized as mild (61%) or moderate to severe (35%). Concordance between diary and electronic data was 85%, with overreporting on diary cards noted on 15% of the total days. Sensitivity of the diary data relative to the electronic data ranged from 0.80 to 0.91 during the 12-week study. Diary return rates decreased from 75% during the initial 3 weeks to 44% at 12 weeks. CONCLUSIONS: Electronic monitoring of nebulizer use provides a more precise measure of long-term medication use than does self-report on diary cards, and it is feasible for use in high-risk populations. However, diary cards seem to be a valid alternative for short-term monitoring of nebulizer use, resulting in only a slight overestimation of medication use.