We present four cases in which probable sexual transmission of Giardia lamblia was suspected. Diagnosing this mode of transmission in endemic areas is often difficult and should be considered only as possible, because exposure to poor sanitation and a potentially contaminated environment are always latent. However, as patients reported, there was no history of drinking tap water, exposure to recreational water, eating contaminated food, or other potential sources of infection but anilingus with an infected partner. We consider that in endemic countries, even when other more frequent modes of transmission could be playing the main role, the possibility of (re)infection due to sexual transmission should not be forgotten. Talking openly with patients, strengthening patient-specific preventive measures and counselling appear to be needed to reduce risks of Giardia infection transmission due to this often neglected route.