To better define the relationship between ovine lentivirus (OvLV) infection and respiratory disease, pulmonary leukocytes and postmortem lung specimens from 42 sheep seropositive or at risk for OvLV infection were obtained. The lungs were examined for lesions of lymphoid interstitial pneumonia (LIP), and animals were categorized into five groups by severity of LIP and OvLV serologic status. The presence of OvLV in alveolar macrophages was established by proviral DNA amplification using the polymerase chain reaction (PCR), and the proportion of infected cells was determined by a quantitative focal immunoassay (FIA) and by immunohistochemistry. The concentration of OvLV p25 in serum was measured by capture ELISA. In contrast to animals with mild or no pulmonary lesions, sheep with moderate or severe LIP (17/42) were all seropositive, 71% had antigenemia (greater than 2 ng/mL), and 82% had proviral DNA in 1.5 x 10(5) alveolar macrophages. Of sheep positive by PCR, those with moderate or severe LIP (79%) had an average of 3 infected cells/10(3) alveolar macrophages by FIA. These results implicate alveolar macrophages as important target cells in the pathogenesis of OvLV-induced respiratory diseases.