The effect of pyrantel pamoate was compared with that of piperazine phosphate in the treatment of ascariasis, the results being expressed in terms of quantitative egg counts. Pyrantel pamoate, in a single oral dose of 10.0 mg per kg of body weight, achieved a cure rate of 90% on day 3 and 100% on day 10 after treatment. Administration of 5.0 mg and 2.5 mg per kg similarly achieved early and pronounced reduction of egg output with a cure rate of 90% by day 10. In patients not cured, mean reduction of egg output was almost complete with the 10.0-mg treatment and 88% with the 2.5-mg treatment calculated throughout days 8, 9, and 10. Standard oral treatment with piperazine phosphate achieved a cure rate of 71% by day 10, significantly less than that with 2.5 mg of pyrantel pamoate. In patients not cured, a mean reduction in egg output of 72% was achieved, as calculated throughout days 1, 2, and 3, with no further improvement. Other than transient abdominal colic in a few patients during the early posttreatment period, no obvious drug-related symptoms were observed. No trend of abnormality was noted in either serum glutamic pyruvic transaminase or alkaline phosphatase levels after pyrantel pamoate treatment.