- Background. Giardia lamblia (G. lamblia) is a prevalent enteric pathogen causing both asymptomatic carriage and diarrheal illness among children worldwide. In this study we examined the hypothesis that G. lamblia may modulate the effect of other enteric pathogens during concurrent infection. Methods. Bedouin (n = 238) infants were followed from birth to ages 18–23 months. Morbidity was ascertained by weekly home interviews and infant's stools were collected fortnightly. The risk for and severity of diarrheal disease were compared between episodes caused by enteric pathogens other than G. lambliaalone and G. lamblia-mixed episodes. Results. In 59 of 293(20.1%) diarrhea episodes two or more pathogens were detected. In two thirds of the mixed episodes G. lamblia was involved. Neither the incidence of diarrhea nor the mean diarrheal episode duration differed between follow up months in which G. lambliawas or was not detected, when age specific or age adjusted values were compared. There were no differences in severity of episodes for single or G. lamblia-mixed infections except for rotavirus. Episodes with rotavirus alone were more severe as compared to episodes where co-infection with G. lamblia was found. Conclusions. The results of this study do not confirm the hypothesis that G. lamblia modulates the clinical effect of infection with other enteric agents. However, the findings of the change in severity of infection with rotavirus suggest that this question warrants further investigation.