Background Epidemiological studies of the effects of hormone replacement therapy (HRT) often rely on exposure data and information on past health from self-administered questionnaires. The accuracy with which women report current use of HRT and the specific preparation in use is not known. This study aims to compare aspects of self-reported use of HRT and treatment for various conditions with data from general practice prescription records.

Methods Reported questionnaire data on use of HRT were compared with those on the general practice prescription record for 570 women participating in the Million Women Study from two general practices in the UK.

Results There was excellent agreement between data from the self-administered questionnaire and the prescription record: 96% agreement ( κ= 0.91) for current use of HRT, 95% agreement ( κ= 0.90) for any use of HRT during the period covered by the prescription record, and 97% agreement ( κ= 0.95) among current users for whether the HRT preparation contained oestrogen alone, combined oestrogen/progestogen, or some other constituents. Among former HRT users who provided questionnaire information on the preparation they used most recently, there was 69% agreement on the proprietary preparation used and 97% agreement ( κ= 0.93) on the hormonal constituents used. Agreement between reported treatment for various conditions and the presence of a prescription appropriate for that condition ranged from 89-99% ( κ0.53-0.92), and was highest for thyroid disease and asthma.

Conclusion Important aspects of use of HRT, such as type of preparation currently being used, are reported very reliably by women completing a self-administered questionnaire.