In a German multicentre study (1603 patients, 320 private practices), adverse effects and patient compliance during antihypertensive therapy were investigated using standardized questionnaires for both patient and physician. Patients with a change in antihypertensive therapy during the last six months were included in this study. The single most important reason for the change in therapy was inadequate blood pressure control (48.4%), followed by adverse effects (30.1%), patient dissatisfaction (20.0%), non-compliance (16.8%) and cost (4.9%). The most frequent adverse effects noted by the doctors were cough (51.9%), oedema (36.9%), flush (36.6) and dizziness (27.8%). In comparing the answers of the physicians and patients, it becomes obvious that compliance may be overestimated by the doctors (good: 41.7%; medium: 57.3%; bad: 1.0%), since only 32.3% of the patients stated that they never missed a dose, 54.8% were occasionally non-compliant and 12.9% admitted missing a dose frequently. The predominant reasons for non-compliance (assessed by the patients) were forgetfulness (40.4%), followed by adverse effects (9.6%) and irregular lifestyle (6.5%). Thus, lack of effectiveness and adverse effects/patient dissatisfaction/non-compliance contributed roughly equally to the decision to change therapy. In addition, forgetfulness was shown to be an important contributor to suboptimal compliance. Lastly, physicians may still underestimate the extent of non-compliance.