Disruption of periciliary fluid homeostasis is the main pathogenesis of otitis media with effusion (OME), one of the most common childhood diseases. Although the underlying molecular mechanisms are unclear, it has been suggested that the altered functions of ion channels and transporters are involved in the fluid collection of middle ear cavity of OME patients. In the present study, we analyzed the effects of a major cytokine interleukin (IL)-1beta, which was known to be involved in the pathogenesis of OME, on Na(+)-K(+)-2Cl(-) cotransporter (NKCC) in human middle ear cells. Intracellular pH (pH(i)) was measured in primary cultures of normal human middle ear epithelial (NHMEE) cells using a double perfusion chamber, which enabled us to analyze the membrane-specific transporter activities. NKCC activities were estimated by the pH(i) reduction due to bumetanide-sensitive intracellular uptake of NH(4) (+). In NHMEE cells, NKCC activities were observed only in the basolateral membrane, and immunoblotting using specific antibodies revealed the expression of NKCC1. Interestingly, IL-1beta treatments augmented the basolateral NKCC activities and increased NKCC1 expression. In addition, IL-1beta treatments stimulated bumetanide-sensitive fluid transport across the NHMEE cell monolayers. Furthermore, an elevated NKCC1 expression was observed in middle ear cells from OME patients when compared to those from control individuals. The above results provide in vitro and in vivo evidence that the inflammatory cytokine IL-1beta upregulates NKCC1 in middle ear epithelial cells, which would be one of the important underlying mechanisms of excess fluid collection in OME patients.