Recurrent viral infections are frequently observed in children with atopic asthma. In this study we investigated the ability of the synthetic immunomodulator pidotimod to affect in vitro the phenotype and/or cytokine profile of blood cells in relation to atopic asthma. Peripheral blood mononuclear cells were isolated from 13 atopic asthmatic and 9 normal children and stimulated in culture with mitogen either in the presence or not of the drug. Expression of surface markers was evaluated by flow cytometry, and production of interleukin-4 and interferon-gamma was measured in supernatants. Pidotimod was able to down-regulate the expression of CD30 on cells from both atopic and normal subjects. Because CD30 has been associated with Th-2 cells, this observation supports the possibility of pidotimod being able to affect the Th-1/Th-2 balance in atopic asthma.