Infection with giardia may be associated with significant ill-health and while the reported incidence of infection is increasing in the United Kingdom, the true prevalence of infection and extent of morbidity due to this organism is unknown. Diagnosis is made difficult by non-specificity of symptoms and low sensitivity of traditional diagnostic techniques. Immunological methods of diagnosis hold promise for the future, but in the meantime, more routine testing by laboratories and multiple faecal testing by clinicians may prevent unnecessary morbidity. The late summer/autumn peak in reported infection is difficult to explain while the age distribution is typical of an organism which is spread faeco-orally. The importance of potable water supplies as a source of infection in this country is not clear, nor is the role of zoonotic spread. The apparent susceptibility to infection of certain population groups requires further exploration as does the role of the asymptomatically infected in transmission.