We have studied 70 children, born to HIV seropositive mothers, since the first trimester of life and every three months thereafter. The virological markers we used in the diagnosis included: 1) p24 antigen detection. 2) Autochthonous production of antibodies detected by Western Blot technique. 3) HIV isolation. 4) Specific determination of IgM antibodies. In infected children under 15 months of age, p24 antigen was positive in 78%, HIV was isolated in 75% and autochthonous production of antibodies occurred in 50%. IgM specific antibodies were detected in 92%, but these were also detected in the 33% of the children who seroreverted. In seroreverted children, the other three virological markers were negative. The problems due to the low sensitivity in p24 antigen detection, HIV isolation and the detection of autochthonous production of antibodies, as well as the low specificity of the IgM detection, means that it is necessary to simultaneously use several techniques in the diagnosis of these children.