This report describes a case of two patients who were admitted to the Zadar hospital and according to clinical symptoms directed to the Department of Lung Diseases. Both patients were temporarily employed abroad. It has been established that they were infected with human immunodeficiency virus type 1 (HIV-1). One of the patients has been moved to the Department of Infectious Diseases and later to Zagreb, while the other has returned abroad. On admission to the hospital of the Zadar Medical Center none of them answered the question about being engaged in risky behavior. In 1990 there were 699 registered patients hospitalized and 745 registered in the protocol of the Outpatient Clinic of the Department of Lung Diseases. 0.069% of patients were HIV-1-infected. In 1991, there were 520 hospitalized and 453 outpatients, whereas 0.102% were HIV-1-infected and registered subjects. It must be pointed out that these are only numbers of registration and not subjects, because there were patients who were examined or hospitalized twice or more times during the corresponding calendar year. The aim of this study was to point to a new differentially-diagnostic problem present especially at the Department of Lung Diseases after AIDS has become part of our reality. There still remains a problem in regard to detection of HIV-1 seropositivity in patients at departments with opportunistic infections such as tuberculosis.