The identification of factors that contribute to noncompliance with trial drug initiation where virtually complete compliance might be expected, may help identify patients whose management is least likely to comply with clinical guidelines and study protocols.Methods: Examination of cross-sectional and longitudinal data arising from the Second Australian National Blood Pressure Study. Prevalence rate ratios (RR) and 95% confidence intervals (CI) estimated from log-binomial models were used to assess associations between subject characteristics and whether the randomized drug was prescribed at trial randomization. The study population consisted of 6083 hypertensive Australians aged 65 to 84 years.Results: After adjusting for each variable in a multivariate model the following were significant predictors of failure to prescribe RR (95% CI): not previously prescribed antihypertensive drugs 2.89 (2.52–3.32); lower systolic blood pressure (BP) 1.51 (1.59–1.43) or diastolic BP 1.18 (1.22–1.13); younger age 80 to 84 v 65 to 79 years 0.75 (0.59–0.95); total cholesterol 6.6 v 5.0 mmol/L 1.27 (1.06–1.53); and not ever having had a raised cholesterol 1.20 (1.04–1.40). There was no difference in cardiovascular disease end points and death in those who were and were not initiated on drug therapy at randomization.Conclusions: Failure to initiate antihypertensive therapy for patients at the randomization visit was associated with lower BP measurements, previous drug therapy, younger age, and a historic or current raised cholesterol, but not with overt cardiovascular disease or other risk factors. There was no difference in cardiovascular disease end points and death in those who were and were not initiated on drug therapy at randomization.