BACKGROUND: International recommendations such as the CONSORT and International Conference on Harmonisation statements recognize patient adherence to prescribed treatment as an important aspect of a treatment's evaluation, but this issue is little assessed. OBJECTIVES: To evaluate how medication adherence was assessed and reported in recently published randomized controlled trials (RCTs). MATERIAL AND METHODS: All publications of RCTs assessing pharmacological treatments in 6 major chronic diseases published in high-impact-factor journals in 2003 and 2004 were selected from the Medline database. Two investigators analyzed how medication adherence was assessed and reported. RESULTS: A total of 192 publications were analyzed: 71 in HIV infection, 48 diabetes mellitus, 24 rheumatoid arthritis, 23 asthma, 15 hypertension, 7 osteoporosis, and 4 about 2 of these diseases. The assessment of medication adherence was documented in 69 (35.9%) publications, by counting pill intake in half of these. Results of adherence were reported in 64 (33.3%) publications. Adherence was reported as a quantitative measure: Proportion of the treatment prescribed in 27 articles and as a qualitative measure (adherent patient, yes/no) in 41 (in 4 reports both techniques were used). When reported, the median intake of prescribed medication was 93%, and the median proportion of "nonadherent" patients was 6.2%. CONCLUSIONS: There is important variability in the assessment and reporting of medication adherence in published RCTs of pharmacological treatments of selected chronic diseases, for a given disease and across diseases. Standardization is advisable to allow for comparisons among studies.