Within 4 days, two oncology patients in the Clinical Research Unit had bacteremia caused by group Y Neisseria meningitidis. Three additional patients, identified by prevalence survey, were found to have nasopharyngeal colonization with that serogroup, compared with only one employee (not associated with patients) and no family contacts. Infected and colonized (case) patients were located in the same or adjacent rooms but did not have close contact. A comparison of host and risk factors showed no significant differences between case patients and the other (control) patients located in the same rooms. In retrospect, the index patient, who had marked sputum production but was not isolated, had unrecognized meningococcal pneumonia and probably was the hospital source of a heavy, airborne dispersal of organisms to other patients. Respiratory isolation is warranted for patients with suspected N. meningitidis infection.