A clinical study was carried out in 23 patients with borderline hypertension. In order to verify the clinical effects of Chlorella on hypertension, thirteen patients were give 1.5g of chlorella daily for 6 months (Chlorella group) and the other 10 patients were given health advice only (control group). The chlorella used for the study, was cultured under heterotrophic conditions with glucose as the carbon source. A significant reduction in both systolic and diastolic blood pressure was observed after 3 months in the chlorella group, while no significant fluctuations in the blood pressure of the control group were observed during the study period. After 6 months, systolic and diastolic blood pressure in the chlorella group were reduced by 14.1 +- 1.9 and 6.3 +- 1.8 mmHg, respectively. The results of a questionnaire answered by the Chlorella group also indicated a reduction of hypertension-related symptoms, and improvements in general physical condition. A variety of clinical data showed that Chlorella ingestion had no harmful effects on the subjects. These results suggest that daily ingestion of Chlorella is effective in reducing hypertension