Changes identified by inferential statistics from summer to winter of high to low arterial blood pressures (BP) have been quantified as a large predictable about-yearly BP swing. This condition of a large annual BP amplitude (LABPA) raises concern about hypotension as well as hypertension and raises new questions regarding appropriate guidelines for diagnosis and treatment. Recommendations made in the fall on the basis of data collected in the summer may be totally inadequate in dealing with the patient's condition in the winter. In order to avoid such mistakes, it is imperative to implement a systematic surveillance of BP in the light of current chronobiological limits. Patients with a large circannual BP amplitude are particularly suited for a study of the underlying hormonal mechanisms. The longitudinal monitoring of their BP is also amenable to the study of environmental influences from near and far.