OBJECTIVES: This study performed meta-analysis examining the correlations of measurements of medication adherence (medication event monitoring system [MEMS] and self-reported questionnaires) using published study reports. METHODS:   The literature search was performed from 1980-2009 using PubMed, PubMed In Process and Non-Indexed, OVID MEDLINE, PsycINFO (EBSCO), CINAHL (EBSCO), OVID HealthStar, EMBASE (Elsevier), and Cochrane Databases. The following search terms were used: patient compliance, medication adherence, treatment compliance, drug monitoring, drug therapy, electronic, digital, computer, monitor, monitoring, drug, drugs, pharmaceutical preparations, compliance, and medications. Studies were included if the correlation coefficients [Pearson correlation coefficient (rp) or Spearman rank correlation coefficient (rs)] between adherences measured by both MEMS and self-reported questionnaires were available or could be calculated. A meta-analysis was conducted to pool the correlation coefficients using random-effect model. RESULTS:   A total of 11 studies (N=1684 patients) met the inclusion criteria. The mean of adherence measured by MEMS was 74.9%, ranging from 53.4% to 92.9% and 84.0% by self-reported questionnaire, from 76.5% to 93.5%. The correlation between adherence measured by MEMS and self-report questionnaires ranged from 0.24 to 0.87. Seven (63.6%) articles reported rp values whereas 4 studies (36.4%) had rs values. The combined effect size for 11 studies was 0.45 (p<0.001, 95% confidence interval [95% CI]: 0.34-0.56). The subgroup meta-analysis on the studies reporting rp and rs correlation revealed the effect size 0.46 (p<0.001, 95% CI: 0.33-0.59) and 0.43 (p<0.001, 95% CI: 0.23-0.64), respectively. CONCLUSIONS: The correlation between adherence measured by MEMS and self-reported questionnaires tends to be moderate. Therefore, self-reported questionnaires give good estimate of patient medication adherence. However, MEMS and self-reported questionnaire should be used complementary to get accurate measure for patient adherence.