An elevation of systolic and diastolic blood pressure to values regarded as abnormal ones on the basis of conventional criteria was recognized by self-measurement. For both systolic and diastolic blood pressure, the overall means adjusted for rhythms, the so-called mesors, were elevated in the light of their response to treatment. These mesors were found to be lowered with statistical significance when values during treatment were compared by an objective test with values measured before treatment. Individualized rhythmometry quantitatively characterizes a predictable portion of the variability in human blood pressure and tests for the statistical significance of changes in blood pressure as a function of the treatment and also as a function of the circadian timing of such treatment. The case report thus illustrates an individualized chronotherapy of systolic and diastolic mesor-hypertension, diagnosed retrospectively from the tested effect of hydrochlorothiazide. In the case reported, and perhaps routinely, computer-analyzed self-measurements can serve to prescribe the right kind and amount, with the right timing, for a given therapy, and can serve for diagnosis and prevention.