Background: Statin treatment is associated with a reduction in the risk of cardiovascular events. To be effective, adherence is essential but this may be influenced by the occurrence of a cardiovascular event while being on treatment. Objectives: To assess the effect of an apparent first event on statin adherence rates in type 2 diabetes patients. Methods: A matched cohort study was conducted among type 2 diabetes patients initiating statin treatment for primary prevention in the Groningen University IADB.nl pharmacy database. Index patients who had a drug-treated cardiovascular event (index date) after statin initiation were matched to a reference patient without such an event with similar gender, age at statin initiation, initiation date, follow-up period and adherence level before the event. Medication proxies were used to identify apparent events. Adherence rates were measured as percentages of days covered (PDC), relative risks with 95% confidence intervals (95%CI) were calculated for shifts in adherence levels (nonadherent, partial-adherent, full-adherent) around the index date. Adherence rates around the index date were evaluated using Wilcoxon signed ranks tests and Mann-Whitney U Tests. Results: We could match 375 of the 855 eligible index patients. Index patients had on average a PDC of 81% after the index date; reference patients had a PDC of 71% (P