In January 2018, the Wisconsin Department of Health Services, Division of Public Health (DPH), received a report of a culture-confirmed case of Legionnaires' disease. The patient, who was immunocompromised, had died at a local hospital 10 days after being admitted. DPH and an infection preventionist from the hospital investigated to determine the source of the infection and prevent additional cases. Because the case was suspected to be nosocomial, health care facility water samples were tested for Legionella. When these samples were negative, water sources in the patient's home were tested. These tested positive for Legionella pneumophila, and the bacteria remained after an attempt to remediate. The patient and home isolates were identified as L. pneumophila serogroup 3, sequence type 93, by whole-genome multilocus sequence typing. A second resident of the home did not become ill. This case highlights the potential for immunocompromised persons and others at risk for Legionnaires' disease to be exposed to Legionella through home water systems containing the bacteria and demonstrates the difficulty of home remediation. This case also illustrates the role of lower respiratory tract specimens in the identification of less common Legionella infections (e.g., L. pneumophila serogroup 3) and confirmation of the infection source.