In order to study the prevalence, incidence, and symptoms of infections with Giardia lamblia and Entamoeba histolytica, we followed 251 expatriates in Bangladesh over a 1-year period. Microscopic examination of fecal specimens was performed upon enrollment, at 3-month intervals, and during episodes of diarrhea. Specimens were cultured for bacteria and samples of serum and saliva were collected for antibody studies (IgG and SIgA). The prevalence of G. lamblia infections was 5.2% and the incidence 11.8%. Children aged less than or equal to 10 years and newcomers were most frequently infected (P less than 0.02). Symptoms were present in 37% of the subjects infected with G. lamblia. A systemic antibody response was observed in 57% of symptomatic patients and 35% of asymptomatic subjects during the first 2 months of infection. The prevalence and the incidence of E. histolytica infection were 3.2% and 8.6%, respectively. Infections with E. histolytica were correlated with the duration of stay in Bangladesh; less than 1 year 7% vs. greater than 3 years 26% (P less than 0.01). Most expatriates infected with E. histolytica were asymptomatic (90%). Three adult patients, who were resident in Bangladesh for less than 1 year, were symptomatic but none of them developed dysentery or a serological response. Four of 25 asymptomatic subjects had significant antibody titers. Three of these people were seropositive for ameba at the beginning of the study. The local immune response, reflected by specific secretory IgA in saliva samples, correlated poorly with both E. histolytica and G. lamblia infections.