A personal diary may be more appropriate than a questionnaire for assessing self-reported current use of hormone therapy (estrogens, progestagens, or their combination); however, use of a questionnaire is more feasible and less expensive. The authors compared both methods for 16,060 Swedish women aged 45-73 years from the Malmo Diet and Cancer Study (baseline, 1991-1996). In a reliability analysis, the authors investigated the agreement (kappa value) between the questionnaire and the diary regarding current hormone therapy use (yes vs. no), studying the ability to replicate results whether or not they were correct. They also explored associations between discrepancy and individual characteristics. A validity analysis was conducted to determine whether use of the questionnaire achieved an outcome without systematic error (i.e., high specificity and sensitivity); the personal diary was considered the "gold standard." Agreement between both methods was high: 95.5% (kappa = 0.840). The sensitivity was 84.9% and the specificity 97.7%. Higher body mass index and being a widow were associated with agreement, whereas age (50-59 years), use of anxiolytics/hypnotics or opiates, high alcohol consumption, past smoking, and higher educational level were associated with discrepancy. Compared with a personal diary, a simple self-administered questionnaire is a valid method for assessing current use of hormone therapy.