OBJECTIVES: Montelukast has been approved for the prophylaxis and chronic treatment of asthma in patients 12 months and older, acute prevention of exercise induced bronchospasm in patients 15 years and older and relief of symptoms of seasonal and perennial allergic rhinitis in patients as young as 6 months. We examined trends in montelukast use, new use, and on-label and off-label use from 1998 to 2008. METHODS: In this retrospective cohort study of administrative claims, eligible subjects in the Lifelink database between January 1997 and December 2009 were analyzed.  Study subjects had to be continuously enrolled for two years.  On label use was determined by searching for ICD-9-CM codes for approved indications; asthma, exercise induced bronchospasm, and allergic rhinitis.  OLS regression was used to assess linear trends over time. RESULTS: 0.16% of subjects were prescribed montelukast in 1998 and 2.44% in 2008 and the linear trend of increase in use was significant (R2=0.978, p<0.0001). 37,793 subjects were eligible for the analysis of off-label use with a mean age of 31.22 years and 43.6% were male. Off-label use was lowest in 1999 (12.56%) and highest in 2007 (24.54%) with a significant increasing trend (R2=0.5776, p=0.0066) but it increased more modestly from 2002 (20.6%) to 2008 (23.91%).  Overall 23.02% of subjects used montelukast off label and the elderly (31.45%) and persons from the south (28.16%) had the highest rates of montelukast off label use. CONCLUSIONS:  Montelukast prescribing increased substantially over the previous decade where in 2008, 1 in 40 persons with commercial insurance had at least one prescription filled.  Over three quarters of montelukast prescribing is supported by a diagnosis for a labeled indication. Off label use is greatest in the elderly which may be partially explained by more evidence based off label uses in the literature for adults.