The passive intestinal permeability of patients seeking care for acute diarrhoea was investigated with a liquid meal containing differently sized, low-molecular weight polyethylene glycols (PEG 400; mol. wt. 282-590). The subjects suffered from acute infections caused either by Giardia lamblia or rotavirus. The patients were studied during infection and 3-4 weeks later when they had recovered clinically. It was found that both giardia and rotavirus infections were associated with decreased 6-h urinary recovery of the PEG molecules, particularly of the larger molecular weight species. After the infection, the permeability properties returned towards normal values. The results show that the permeability and the absorptive capacity is altered in patients with acute Giardia lamblia and rotavirus infections which could be important in relation to chronic infections and malnutrition attributed to these organisms.