A study on the reinfection and infection rates of soil-transmitted helminths was conducted in Kemiri Sewu, Godean District, Sleman Regency, Yogyakarta Special Province, Indonesia, from August 1988 to June 1990. A total 61 individuals rendered Ascaris lumbricoides egg-negative by drug treatment were used in the reinfection study and 412 individuals found negative for Ascaris eggs in the pretreatment stool examination were used for the infection rate study. In both groups, children had higher and faster reinfection and infection rates than adults. The start of reinfection was 4 months after treatment, and the start of infection was at the 1st month. At 20 months after treatment, the reinfection rate of ascariasis was 14.8% and the infection rate was 11.4% among both children and adults. There were 140 subjects rendered Trichuris trichiura egg-negative by drug treatment, and 317 Trichuris egg-negative at the pretreatment stool examination which were used in the reinfection and infection rates studies. In general, the reinfection rates of T. trichiura among adults were higher than those among children. The start of reinfection was 4 months after treatment, and the infection was at the 1st month. At the end of 20 months, the reinfection rate was 36.4% and the infection rate was 21.8% among both children and adults. A total 83 subjects rendered Necator americanus egg-negative by drug treatment and 365 N. americanus egg-negative at pretreatment stool examination were used in the reinfection and infection rate studies. Throughout the study, adults had always higher reinfection and infection rates than children, while adults males had higher reinfection rates than adults females. The start of reinfection was 4 months after treatment, and the start of infection was at the 1st month; they were 25.3% and 9.3% respectively. At the end of 20 months, the reinfection rate was 65.1%, and the infection rate was 47.9% among both children and adults.