A seroepidemiologic survey was done to ascertain the level of immunity in a population of hospital employees after contact with patients with Legionnaires' disease. Two matched groups were compared: hospital staff in positions of contact with patients diagnosed with the disease (N1 = 215), and hospital staff not in a position of contact with patients diagnosed with Legionnaires' disease (N2 = 269). Antibody titer was measured by the hemagglutination technique. Subjects from N1 and N2 were surveyed for age, sex, race, smoking, patient care unit, airconditioning unit, occupation, symptoms, and patient contact. No significant correlation was found between titer distribution and any one of the first seven factors. The prevalence of antibody (≥ 128) was 9.3% and 3.7% (P < 0.02) for the N1 and N2 groups. Also, 40% of employees with titers of 128 or above had had an unexplained febrile respiratory illness in the preceding year. This study suggests the possibility of person-to-person transmission in Legionnaires' disease.