Eleven cases of legionnaires' disease, all patients living in the same municipality, were admitted to a district hospital in northern Portugal. Preliminary investigations suggested an association with the events of the annual festivities in that municipality. Legionella pneumophila was not isolated from any of the suspected sources, but evidence from a case control study suggested that an aerosol produced by a decorative fountain in the main square during the night of a rock concert was the likely vehicle of infection. The prevalence of smoking was higher among cases than controls.