Introduction. In clinical practice, end-digit preference is a common feature of blood pressure (BP) measurements. A wider use of electronic BP measuring machines could decrease this observer-linked artefact. The purpose of this analysis was to investigate the frequency of end-digit preference and to evaluate the impact of this observer bias on the assessment of the BP control induced in a large group of hypertensive patients treated with a calcium-channel blocker in whom BP was measured either with an automatic device or with a conventional sphygmomanometer. Methods. Five hundred and four physicians participated in the study and 2199 patients were included. Treatment with lercanidipine was introduced at a dosage of 10 mg and titration to 20 mg was optional according to the physician's decision. BP was assessed at 4 and 8 weeks. To measure BP, physicians could use either a standard mercury sphygmomanometer or a pre-defined validated semi-automatic device (Microlife Average Mode, BP 3AC1-1, Microlife Corporation, Berneck, Switzerland) but they had to use the same method throughout the study. Physicians had to transcribe all BP measurements onto case report forms. Results. Very marked digit preferences were observed for both the conventional and the automatic measurements, being most prominent for the digit "0" (52% and 25%, respectively) followed by a preference for the digit "5" (19% and 15%). The use of the semi-automatic device reduces to a certain extent the frequency of the bias but the problem remains if physicians have to transfer the BP values onto case report forms. The end-digit preference has a major impact on the evaluation of a treatment effect and on the assessment of the percentage of patients achieving target BP in a population. Conclusion. These results confirm that end-digit preference remains a serious bias in clinical practice. This bias has important consequences when evaluating the efficacy of a new antihypertensive drug. There is a need for training programmes and quality controls in clinical practice. The development of automatic systems with a direct transfer of BP values from the measuring device to the clinical chart or to the case report form should be encouraged