The distribution of antibody levels to Legionella (L.) pneumophila (serotypes 1–7) was compared between subjects who worked near the source of a large outbreak of Legionnaires' disease (n=668) and a population sample of comparable age (n=480). In a previous analysis of these data, it was estimated that 80% of those working near the source were infected with L. pneumophila. However, the estimation procedure implicitly assumes that the probability of infection does not depend on the antibody level of a person before exposure. This is questionable, as antibodies could protect against infection. We have now estimated the minimum value consistent with the data on the number of infected persons. We observed that a minimum of 40% [95% confidence interval (CI) 32–48] of those working near the source and 13% (95% CI 8–18) of those working further away were infected with L. pneumophila. Implications of these findings for design options in future research are discussed.