In 70 patients with proven giardiasis the efficacy and cost-effectiveness of the diagnostic procedure were studied retrospectively. In 53% of the patients, complaints existed for more than 6 months. Parasitological examination of three stool specimens on non-consecutive days showed a sensitivity of 95.7%. The false negative rate was 2.8%, as giardiasis was found in 3 out of 109 patients in whom duodenal biopsy was performed after a negative “triple faecal test”. Of other laboratory investigations performed before the results of faecal screening were known, 75% gave normal results, thus not contributing to the diagnosis. A proposal for a cost-effective diagnostic procedure is presented.