OBJECTIVES: We hypothesized that the average medication possession ratio (MPR) for controller medications in adult asthmatic populations may be misleading as a basis for evaluating adherence. METHODS: MPR distributions were determined from analysis of the Pharmetrics database for the period April 2004 to March 2005. We assessed MPRs for those plan members exclusively on one of the three major controller medication groups: inhaled corticosteroids, combined inhaled corticosteroids and long acting beta agonists, and leukotriene modifiers. MPR were defined in terms of annual days supplied (range 0 to 1.0). RESULTS: Overall, for the study population (n = 17,581) 5,806 were exclusively on combination therapy (33.8%), 2,689 were exclusively on leukotriene modifiers (15.3%) and 2,106 exclusively on inhaled corticosteroids (12.0%). Average (median) MPRs were 0.51 (0.57) for combination therapy; 0.55 (0.63) for leukotriene modifiers; and 0.39 (0.33) for inhaled corticosteroids. Distributions of MPRs for these groups showed significant bi-modality (a “U” shaped MPR profile). This characteristic was most pronounced for the combined controller group medications and the leukotriene modifiers where the frequency distributions were almost identical. For the combined medications 32.2% of adults reported a MPR <0.2, with 19.0% and 28.9% reporting MPRs of 0.6 to 0.79 and 0.8 to 1.0 respectively. The corresponding estimates for leukotriene modifiers were 27.5%, 19.6% and 33.1% respectively. For inhaled corticosteroids, 45.2% of patients reported an MPR <0.2 with only 33.2% reporting an MPR > 0.6 (15.5% > 0.8). CONCLUSION: This analysis confirms the hypothesis that average MPR may be a misleading indicator of adherence in patients taking combined controller medications and leukotriene modifiers. We suggest an approach that takes explicit account of the underlying MPR distribution to understand better the nature of asthma controller medication possession.