Since implementation of Standard Precautions* for the prevention of bloodborne pathogen transmission in 1985, health care-associated transmission of human immunodeficiency virus (HIV) in the United States has been rare (1). In October 2017, the New York City Department of Health and Mental Hygiene (NYCDOHMH) and the New York State Department of Health (NYSDOH) were notified by a clinician of a diagnosis of acute HIV infection in a young adult male (patient A) without recognized risk factors (i.e., he was monogamous, had an HIV-negative partner, and had no injection drug use) who had recently been hospitalized for a chronic medical condition. The low risk coupled with the recent hospitalization and medical procedures prompted NYSDOH, NYCDOHMH, and CDC to investigate this case as possible health care-associated transmission of HIV. Among persons with known HIV infection who had hospitalization dates overlapping those of patient A, one person (patient B) had an HIV strain highly similar to patient A's strain by nucleotide sequence analysis. The sequence relatedness, combined with other investigation findings, indicated a likely health care-associated transmission. Nucleotide sequence analysis, which is increasingly used for detecting HIV clusters (i.e., persons with closely related HIV strains) and to inform public health response (2,3), might also be used to identify possible health care-associated transmission of HIV to someone with health care exposure and no known HIV risk factors (4).