OBJECTIVE: To investigate the reasons for withdrawal from double-blind randomized trials, and the reasons for changing treatment within a randomized therapeutic group. DESIGN: The Syst-Eur trial, in which 4695 older patients with systolic hypertension were randomized to active or placebo treatment. METHODS: The reasons for withdrawal from the trial were examined, both for patient-initiated and investigator-initiated withdrawals. In addition, the reasons for stopping the first-line treatment (nitrendipine), the second-line treatments (enalapril and hydrochlorothiazide) and the corresponding placebos, were determined. RESULTS: A total of 135 patients (6%) were withdrawn by the investigators from placebo treatment because their blood pressure was too high, and, similarly, 36 (1.6%) through patient initiation. The corresponding results for the actively treated patients were 14 (0.6%) and 7 (0.3%). Very few patients were withdrawn from the trial because of the adverse effects of treatment. However, 39 (4%) stopped taking active nitrendipine because of ankle oedema, compared with 4 (0.5%) on placebo. Similarly, 28 versus three stopped due to flushing. Forty-one (10%) stopped taking enalapril because of cough, against eight (2%) for enalapril placebo. In all, 15.0% stopped active nitrendipine, 20.2% enalapril and 6.3% hydrochlorothiazide, versus placebo 7.1, 9.1 and 5.1%. CONCLUSIONS: The numbers withdrawn from the trial for adverse treatment consequences were small in comparison to the cardiovascular benefits. Nevertheless the numbers stopping individual treatments were higher than expected.