Circulatory complications during the intraoperative and postoperative phases were evaluated in a prospective study on the effectivity of different modes of treatment of hypertonics. 151 patients were subdivided into four groups: Group 1: Treated hypertension, blood pressure normal Group 2: Treated hypertension, blood pressure levels above 160 mmHg Group 3: Untreated hypertension or treatment of hypertension discontinued Group 4: Patients with threshold hypertension. The results show that patients treated inadequately or for a short period only, had the highest complication rates. On the whole, more than 36% of the patients treated inadequately (group 2) during the intraoperative and postoperative periods, had complications of the cardiovascular system, whereas in the untreated patients (group 3) the rate of complications was 30%. Patients with a normal blood pressure (group 1) and with threshold hypertension (group 4), on the other hand, showed a markedly lower incidence of complications, namely, 16%. These results suggest that it would be advisable to effect a consistent preoperative treatment, thus lowering the involved risk. On no account should any possible difficulties in organizing such treatment result in discontinuance of treatment of in-non-treatment.