On March 1, 2010, the Washington State Department of Health (WADOH) notified Public Health - Seattle & King County (PHSKC) of a suspected case of contact transmission of vaccinia virus from sexual contact with a member of the military who had been vaccinated against smallpox. Vaccinia virus infection after sexual contact has been reported previously (1-4). Despite the patient's exposure history and clinical presentation, the diagnosis initially was not considered by the patient's physician, who ordered laboratory testing for several common sexually transmitted infections. The patient was seen by a second physician and referred to an infectious disease specialist, who obtained a swab sample of a genital lesion for laboratory testing for vaccinia virus. Vaccinia virus was confirmed by the Washington State Public Health Laboratory (WAPHL) and the CDC Poxvirus Laboratory. The patient resided in a household with an immunosuppressed renal transplant recipient. Appropriate contact precautions were recommended to the patient. No additional cases of contact transmission were reported. This report describes the patient's clinical course and the associated epidemiologic investigation. Health-care providers caring for U.S. military personnel or their contacts should consider vaccinia virus infection in the differential diagnosis of clinically compatible genital lesions. Contact precautions should be emphasized to all persons who are vaccinated, as well as their contacts with unexplained lesions that might represent vaccinia infection from contact transmission.