The in vitro effect of ibuprofen (IB) on the production of the proinflammatory cytokines interleukin (IL) 1β, IL-6, and tumor necrosis factor alpha (TNF-α) and the anti-inflammatory cytokine IL-10 by cord blood mononuclear cells from preterm newborns was compared to that of peripheral blood mononuclear cells of adults. Mononuclear cells were incubated without or with lipopolysaccharide in the absence or presence of various concentrations of IB. The levels of IL-1β, IL-6, TNF-α, and IL-10 in the supernatants were tested by ELISA. The mononuclear cells from the two groups responded to IB by an increased secretion of IL-6 and TNF-α and by a reduced production of IL-10. The pattern of response to the drug was similar following stimulation with lipopolysaccharide. The IL-1β production was mostly unaffected by IB. It is suggested that in preterm newborns the differences observed in the in vitro proinflammatory cytokine production in response to IB, as observed in the present study, or to indomethacin, as reported previously, may affect various clinical outcomes using these two drugs.