OBJECTIVE: This study examines the predictors of patient persistency for major drug categories related to the treatment of Asthma and COPD. METHODS: The study design was retrospective covering the 12-month, longitudinal period (October 2003- September 2004). It included analysis of electronic CMS1500 claims for 26,152 unique patients (HIPAA compliant). Patients' medication was assessed through NCPDP electronic claims utilizing NDC codes and drug classes. Treatment persistency was identified and compared across drugs in each disease group. Analysis included calculations of descriptive statistics and regression modeling. RESULTS: On average, COPD patients are more persistent then asthma patients. In particular, the persistency of COPD patients taking Advair, Combivent, Flovent and Pulmicort was higher then those patients diagnosed with asthma, respectively by 13.5%, 34.3%, 25.8% and 34.6% (P<0.05). The age of patients is one of the most important predictors of a patients' persistency. For example, the likelihood of refilling a long-acting bronchodilator (LABA) prescription for asthma in patients age 18 years and older is 1.51 times higher (p<0.05) than for those patients who are in the age category 17 years and younger. For both, asthma and COPD, there is a strong relationship between therapeutic class of drugs and persistency. The highest persistency of asthma patients was identified for those who use xanthines, and the lowest for those who take oral beta-blockers (1.89 times difference, P<0.05). COPD patients were more persistent in their use of xanthines and had their lowest persistency on inhaled corticosteroids (1.66 times difference, P<0.05). Logistic regression models were created to quantify the relationship between annual average lengths of treatment and predictors. CONCLUSIONS: There is a significant difference between drug persistency for patients with asthma and COPD even when controlling for the same medication. Adjusted for diagnosis, the major predictors of patient persistency appeared to be patient's age and therapeutic drug category.