To determine the incidence of Loeffler's syndrome in a tropical region, a survey was conducted over a 1-year period in two locations in Colombia, South America, one inland and urban (Cali), the other rural and coastal (Guapi). Among 700 hospitalized patients, 12,000 patients coming to health centers, and 44 families with 328 members, 4 cases of Loeffler's syndrome were found—3 during the survey and 1 before it began. Only one was typical and of some severity. None of the family members developed a perceptible reaction to larvae, though surveillance was close and 83 new intestinal infections were documented. Infection rates for intestinal ascariasis were 25% to 30% in Cali, and 80% to 90% in Guapi. When infections in 30 families were eliminated, previous levels of infection were reached within 10 months. When undisturbed by treatment, the levels of intestinal infection remained stable or showed fluctuations which were not linked to climatic changes. In view of the high prevalence of infection and year-round transmission, which was assumed to occur frequently in the population, it is noteworthy that symptomatic pulmonary ascariasis appeared to be rare and complications from adult worms in heavy infections were uncommon. The findings suggest that frequent and uninterrupted contact between Ascaris lumbricoides and its host results in a high degree of natural tolerance and control.