Abstract he prevalence of the pinworm, Enterobius vermicularis, was determined in 4 provinces of lower northern Thailand (Phitsanulok, Uthai Thani, Kamphaeng Phet, and Nakhon Sawan) during January 2008-May 2010. Of the 1,131 children tested, 19.9% were positive for E. vermicularis eggs by Scotch-tape technique. The infection rate was highest among residents of Phitsanulok (25.0%), followed by Kamphaeng Phet (20.3%), Uthai Thani (17.9%), and Nakhon Sawan (13.8%). The age group 4-6 years had the highest positive rate. The results indicated relatively high overall infection rates. Mass treatment and educational programs should be undertaken, to reduce re-infection in these areas.