In a peri-urban shanty town in Lima, Peru that was hyperendemic for Giardia lamblia44 children aged between 0·9 months and 10 years were effectively treated for Giardia lamblia with tinidazole. Stools were examined weekly in the 6 months after treatment to determine the rate of reinfection, and after reinfection stools continued to be examined. 98% of the children became reinfected with Giardia lamblia within 6 months, and after reinfection stool excretion of the parasite lasted a mean (SD) of 3·2 (3·3) months. The children's mean stool pH and their mean stool fat index was unaffected by Giardia lamblicreinfection. Treatment of all symptomless Giardia lamblicinfections in a developing country hyperendemic for the disease is of questionable value because of rapid reinfection. © 1988.