A retrospective analysis of absolute numbers in 802 white blood counts and 396 sedimentation rates of 407 children, admitted between 1973-78, with 9 "classic" infections was done and evaluated for diagnostic usefulness. As diagnostic meaningful it was found: Lymphocytosis in pertussis; lymphocytopenia and slight increased sedimentation rate in measles; nothing particular in mumps; slight increased sedimentation rate in chicken pox; increase in mononuclear cells, particularly atypical lymphocytes and sedimentation rate in infectious mononucleosis; leucocytopenia caused by neutrocytopenia and lymphocytopenia in exanthema subitum (roseola infantum); increased sedimentation rate in scarlet fever; lymphocytopenia and a high sedimentation rate in mycoplasma-pneumonia; leucocytopenia with lymphocytopenia in rubella.