In the Scottish Adjuvant Tamoxifen Trial, patients with operable breast cancer were randomized to adjuvant tamoxifen or to tamoxifen at first recurrence. Sixty-six Tayside patients from this trial were still alive on 1 January 1993, and still living in Tayside. The Medicines Monitoring Unit (MEMO) collects all prescriptions redeemed by patients in Tayside. From trial records, it was determined whether patients should be taking tamoxifen or not, and this was checked against the dispensed prescription data from MEMO for 6 months from 1 January 1993. Twenty-three were correctly confirmed as receiving prescriptions for tamoxifen and 43 confirmed as not. Known changes to tamoxifen prescribing were reflected in the pattern and timing of the prescriptions. Prescription monitoring has confirmed that all patients were redeeming their prescriptions for tamoxifen in this trial of adjuvant tamoxifen. This would tend to support validity of the trial results as well as data on adverse events. It can be used as a method of quality assurance in clinical trials and to correlate adverse events with prescribing history.