Varicella-zoster virus (VZV)-specific antibodies were tested by indirect membrane immunofluorescent technique in sera of 32 immunocompromized children during the first week after onset of varicella. VZV-specific IgM antibodies were found in sera of all but four patients. Three of the four IgM negative patients were followed up, and it was demonstrated that all of them gave a retarded antibody response. They had severe leukopenia and had several relapses of their VZV-infection. The results indicate (a) that the demonstration of VZV-specific IgM antibodies is of diagnostic value even in atypical varicella of immunocompromized patients; and (b) that in cases with retarded antibody response relapses can be expected.