The relationship between adherence to highly active antiretroviral therapy (HAART) and RNA-HIV viral load outcomes has been extensively shown. Although there are different procedures for assessing treatment adherence, there is no ideal method. We present the SERAD (Self-Reported Adherence) questionnaire, a qualitative and quantitative self-reported instrument designed to provide an easier adherence measurement. We also compared the questionnaire to three other methods to evaluate adherence to HAART regimens in HIV-infected patients. Two prospective, observational, longitudinal studies were developed: a single-center pilot study followed by a multicenter study. A total of 530 HIV-infected outpatients was prospectively included, 66 in the pilot study and 464 in the multicenter study. Four methods were used to study adherence to HAART regimens: the SERAD questionnaire, pill count, electronic monitoring, and plasma drug monitoring. Pearson's correlations and Bland and Altman's method were developed. The SERAD questionnaire showed good feasibility and significant validity. Adequate levels of agreement between methods were observed, particularly when adherence was high. Differences increased as adherence fell. Moreover, the questionnaire was completed correctly, the interviewers did not report uncovered aspects, and the information was collected easily. Our results suggest that the SERAD questionnaire is a feasible and useful instrument for assessing adherence to HAART regimens in HIV-infected patients, and makes it possible to obtain reliable qualitative and quantitative information related to treatment adherence.