Poor adherence to medication regimens is a well-documented phenomenon in clinical practice and an ever-present concern in clinical trials. Little is known about adherence to inhaled medication regimens over extended periods. The present paper describes the 2-yr results of the Lung Health Study (LHS) program, which was developed to maintain long-term adherence to an inhaled medication regimen in 3,923 special intervention participants (as measured by self-report and medication canister weight). The LHS is a double-blind, multicenter, randomized controlled clinical trial of smoking intervention and bronchodilator therapy (ipratropium bromide or placebo) for early intervention in chronic obstructive pulmonary disease (COPD). At the first 4-mo follow-up visit, nearly 70% of participants reported satisfactory or better adherence. Over the next 18 mo, self-reported satisfactory or better adherence declined to about 60%. Canister weight classified adherence as satisfactory or better in 72% of participants returning all canisters at 1 yr, and in 70% of the participants returning all canisters at the 2-yr follow-up. Self-reporting confirmed by canister weight classified 48% of participants at 1 yr as showing satisfactory or better adherence. Overusers were 50% more likely than others to misrepresent their true smoking status, suggesting that canister weights indicating overuse may be deceptive. Results of multiple logistic regression analysis indicate that the best compliance was found in participants who were married, older, white, had more severe airways obstruction, less shortness of breath, and fewer hospitalizations, and who had not been confined to bed for respiratory illnesses.(ABSTRACT TRUNCATED AT 250 WORDS)