Cases of giardiasis diagnosed by stool microscopy performed in a single laboratory serving two general practices on Hong Kong Island were reviewed by retrospective analysis of case notes. From 1 September 1992 to 31 August 1993, 95 stool samples from 88 patients were found positive for Giardia lamblia. Case notes were available for analysis of 77 subjects. There were 50 children (mean age 39 months) and 27 adults (mean age 36 years) in a predominantly Caucasian population. Infection was most common in the spring months of February and March and widespread over Hong Kong island. Diarrhoea (47%) and abdominal pain (27%) were the commonest symptoms. Twenty (23%) of the stool samples positive for G. lamblia were obtained from asymptomatic members of 12 families with a symptomatic case. Treating all family members on the basis of a positive stool result for G. lamblia from one member would seem justified in view of frequent person-to-person transmission and the poor diagnostic sensitivity of stool microscopy.