We compared the diagnostic accuracy of microscopical examination of multiple faecal specimens with duodenal juice examination in the diagnosis of giardiasis. Of 292 patients who had stool microscopy and duodenal aspirate, Giardia were identified in either stools or duodenal fluid from 73 patients (25%). Giardiasis was diagnosed in 62 (73%) with the first faecal specimen, but examination of 3 specimens increased the diagnostic yield to 85%. Giardia, however, were found in only 32 of 73 duodenal aspirates examined (44%). This finding is contrary to the widely held belief that duodenal fluid examination is superior to stool microscopy for the diagnosis of giardiasis. The 2 approaches are complementary, however, since Giardia was found in duodenal fluid, only, from 15% of patients.