Mass gatherings are believed to increase the transmission of infectious diseases although surveillance systems have shown a low impact. The Catholic Jubilee was held in Rome, Italy in 2000. We conducted a case-control study to analyse the risk factors of giardiasis among residents. All diseases reported to the laboratory surveillance system from January 2000 to May 2001 were compared with hospital controls concurrently selected in the same season as cases and frequency-matched for age and birth country. Fifty-two cases (44·1%) and 72 controls were enrolled. In the multivariable analysis factors associated with giardiasis among adults were: travelling abroad (OR 24·2, P>0·01), exposure to surface water (OR 4·80, P=0·05), high educational level (OR 3·8, P=0·03). Having a maid from a high-prevalence country was independently associated (OR 2·3) although not statistically significant. This is the only exposure that changed during the Jubilee.