Basal acid output (BAO) and Maximal acid outputs (MAO) were measured 5 years after highly selective vagotomy (HSV) in 21 patients who had previously undergone serial testing of BAO and MAO for up to 2 years after HSV. BAO was found to have decreased from a mean of 1.7 mEq per hr, 1 year after HSV, to 1.4 mEq per hr, 5 years after HSV (not significant). The mean reduction in BAO at 5 years, compared with the preoperative BAO of 8.4 mEq per hr, was 79%. The mean peak acid response to pentagastrin (PAO-Pg) increased from 20.2 mEq per hr at 1 year to 22.6 mEq per hr at 5 years (0.1 greater than P greater than 0.05). The mean reduction in PAO-Pg, compared with the preoperative PAO-Pg of 43 mEq per hr, was 51% at 1 year and 48% at 5 years. Inasmuch as acid outputs seem unlikely to increase further after 5 years, it is concluded that HSV produces a permanent reduction of about 80% in basal acid output and 50% in maximal acid output.