Classifying medication adherence is important for efficiently targeting adherence improvement interventions. The purpose of this study was to evaluate the use of a novel method, group-based trajectory models, for classifying patients by their long-term adherence. We identified patients who initiated a statin between June 1, 2006 and May 30, 2007 in prescription claims from CVS Caremark and evaluated adherence over the subsequent 15 months. We compared several adherence summary measures, including proportion of days covered (PDC) and trajectory models with 2-6 groups, with the observed adherence pattern. In 264,789 statin initiators, the 6-group trajectory model summarized long-term adherence best, whereas PDC summarized less well. We conclude that trajectory models summarized adherence patterns better than traditional approaches and were similarly predicted by covariates.