Four hundred and twenty-seven hypertensive patients in the United Kingdom have so far been admitted to two on-going open multicentre trials, and have been treated with acebutolol for periods ranging from 1 to 6 months. Oral dosages were within the range of 100 mg to 1200 mg/day in divided doses and were increased or decreased to suit individual requirements. The data on 366 patients completing more than 1 month's treatment were analysed. A high significant reduction in mean diastolic pressure was observed. This reduction was progressive up to 6-month's treatment and was the more marked as pre-treatment values were higher. There was a highly significant correlation between pre-treatment mean diastolic pressure and mean fall of diastolic pressure at 6-months' treatment. There was also a highly sigificant association between initial values and response, and between duration of treatment and response. The higher the pre-treatment value, the less likely was non-response. Response also increased as treatment duration increased. Although asthmatic patients were not excluded, only 2 of the 427 patients included in the present study had to discontinue treatment because of the occurrence of airways obstruction; only 1 of these patients had experienced asthma previously.