The survey on soil-transmitted helminthiasis was carried out in the three villages of Asahan Regency, North Sumatra, Indonesia, between October 1981 and December 1982. The prevalence rates of geohelminthiasis proved to be extremely high (average 97%) in the three villages and more than 70% harboured three or more helminths, especially Ascaris lumbricoides, Trichuris trichiura and hookworm. Necator americanus and Ancylostoma duodenale were detected in 61.1% and 52.2% of 835 fecal specimens, respectively. The effects of two anthelmintics, Combantrin and Trivexan, were essentially the same against A. lumbricoides and hookworm, although Trivexan was better than Combantrin against T. trichiura. The kinetic changes of parasitic infection in community after mass treatment were examined. Ten months after the first drug treatment, the incidence of Ascariasis or trichuriasis reverted nearly to pre-treatment level, while hookworm infection rate remained significantly low. Four months after the second mass treatment, the reinfection rate of A. lumbricoides was most prevalent followed by trichuriasis and hookworm infection. The reinfection rate (63.6%) of A. lumbricoides in children was about six times higher than that (10.4%) in adults at four month after the mass treatment. This study indicates that mass treatment with Trivexan at two month intervals for children and four month intervals for adults is necessary for the effective helminth control scheme in highly endemic areas of North Sumatra.