A Nebulizer Chronolog, a portable device that houses a standard nebulizer canister, was used in a unique method to measure compliance with aerosolized medication. Each actuation is tabulated to within 4 minutes of the actual time of usage and can subsequently be displayed in a day-hour-minute format. Of the 19 patients studied for 12 weeks with a cromolyn-like agent, appropriate usage four times a day ranged from 4.3% to 94.8%. Underusage exceeded overusage and ranged from 5.2% to 95% of the study days. Younger subjects and male subjects were less likely to use the aerosol appropriately. Patients failed to write the truth in their diaries with overreporting of appropriate usage more than 50% of the times. Lack of compliance with aerosolized medication represents an important medical issue for the physicians caring for patients with asthma. The Nebulizer Chronolog elicits new insight into the disparity between reported and observed compliance.