The Weibel graticule was used to assess quantitatively histological changes in proximal jejunal mucosal biopsies from patients with Giardia lamblia infections. Most had malabsorption. A group of patients who had mild abdominal symptoms but no intestinal infection and normal absorption were the controls. There were significant differences in mean surface area (SA) measurements between patients with giardiasis and severe malabsorption and controls (P less than 0.001) and infected patients with normal absorption (P less than 0.05). SA measurements correlated significantly with D-xylose excretion results (r = 0.55; P less than 0.01) and daily facal fat output (r = -0.61; P less than 0.001). Significant correlations between duration of symptoms and SA measurements (r = 0.43; P less than 0.05) and D-xylose excretion (r = 0.43; P less than 0.05) in giardiasis suggest that histological and functional impairment are maximal soon after infection and resolve in time. Treatment with metronidazole or mepacrine was associated with a significant increase in SA (P less than 0.05) in patients with severe malabsorption but there was little change in SA in a similar group of patients who received tetracycline. The Weibel graticule was found to be useful in assessing the severity of histological changes and in following changes after treatment.