Epidemic Giardia lamblia infection has usually been associated with contaminated water. In this paper, five proved and one presumed case of G. lamblia infection among homosexual men are reported. The sequential onset of their clinical illnesses and their sexual interrelationships suggest that this outbreak could have been caused by the direct faecal-oral transmission of G. lamblia. Other parasitic diseases with possible venereal transmission have been described, and parasitic diseases should be considered in homosexual men with diarrhoea or other gastrointestinal symptomatology.