This study was designed to investigate the possible association of high colony counts of legionellae from cooling towers and evaporative condensers with Legionnaires' disease outbreaks. We obtained legionellae counts from samples of cooling towers and evaporative condensers that were the likely sources of two different Legionnaires' disease outbreaks and compared these counts with those from cooling towers that were not associated with reports of human disease. Among 675 potential control cooling tower water samples from 258 facilities, 136 facilities had one or more cooling towers that met our criteria for inclusion into the study. Samples taken from buildings where an outbreak had occurred had much higher Legionella counts than did samples from other buildings. Colony counts from the two outbreak-associated facilities were significantly higher than colony counts from other facilities [Wilcoxon Rank Sum Test (Exact), p < 0.01]. The results of the study suggest that, among cooling towers that test positive for the presence of legionellae, higher colony counts are associated with higher risk of Legionnaires' disease.