It is well known that piglets congenitally infected with porcine reproductive and respiratory syndrome virus (PRRSV) can be viremic at birth, and that preweaning mortality due to secondary infections often increases during acute outbreaks of PRRS. Therefore, an immunosuppressive effect of in utero infection has been suggested. The aim of the present study was to characterise the changes of leukocyte populations in piglets surviving in utero infection with PRRSV. A total of 27 liveborn uninfected control piglets and 22 piglets infected transplacentally with a Danish strain of PRRSV were included. At 2 and 4 weeks of age, 21 of 22 (96%) and 7 of 14 (50%) examined infected piglets were still viremic, whereas PRRSV could not be detected in the six infected piglets examined at 6 weeks of age. Flow cytometry analysis was used to determine the phenotypic composition of leukocytes in peripheral blood and bronchoalveolar lavage fluid (BALF) of 2-, 4- and 6-week-old infected piglets and age-matched uninfected controls. The key observation in the present study is that high levels of CD8+ cells constitute a dominant feature in peripheral blood and BALF of piglets surviving in utero infection with PRRSV. In BALF, the average high level of CD8+ cells in 2-week-old infected piglets (33.4±12.6%) was followed by a decline to 7.3±3.0 and 11.1±3.0% at 4 and 6 weeks of age. BALF of control piglets contained 1.6±0.9, 2.3±1.8 and 1.9±0.5% CD8+ cells, only. In peripheral blood, however, the average number of CD8+ cells remained at high levels in the infected piglets throughout the post-natal experimental period (2.8±1.9, 2.9±1.8 and 3.2±1.7×106 CD8+ cells/ml at 2, 4 and 6 weeks, respectively). In the controls, the average levels of CD8+ cells were 0.9±0.2, 1.9±1.7 and 1.6±0.5×106/ml, respectively. Furthermore, the numbers of CD2+, CD4+CD8+ and SLA-classII+ cells, respectively, in peripheral blood, together with the levels of CD2+ and CD3+ cells in BALF were increased in the infected piglets infected in utero compared to the uninfected controls. The kinetic analyses carried out in the present study reflect that in utero infection with PRRSV modulates immune cell populations in peripheral blood and BALF of surviving piglets. The observed changes are characterised by high levels of CD8+ cells supporting an important role of these cells in PRRSV infection. The present results, however, do not support the existence of post-natal immunosuppression following in utero infection with PRRSV.