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Respiratory syncytial virus (RSV) infection is common in young children, but only a few develop severe bronchiolitis. The relationship between bronchiolitis, asthma, and atopy has been debated for a long time, but the pathogenesis of wheezing remains unclear. A Th1 and Th2-type lymphocyte imbalance seems to be involved in asthma and atopic disease. Serum interleukin-12 (IL-12), IL-10, and soluble CD30 (sCD30) levels were measured by enzyme-linked immunosorbent assay (ELISA) in 23 cord blood samples kept frozen since birth: 11 from normal term newborns who several months later were admitted to the hospital with bronchiolitis, and 12 from newborns who did not develop the disease (controls). The study was also performed on 28 additional children (1–16 months old) suffering an episode of acute bronchiolitis. IL-12 was clearly increased in all cases at birth, but newborns who later developed bronchiolitis showed low IL-12 levels in cord blood compared to newborns who did not develop the disease (median 295 vs. 507 pg/mL; P = 0.001). sCD30 levels were also decreased in the first group (15 vs. 26 U/mL; P = 0.007). During episodes of bronchiolitis, a clear rise of IL-12, IL-10, and sCD30 was observed. None of the factors studied in the acute phase showed statistical differences in children who were later readmitted to the hospital due to repeated wheezing crises. Children who develop acute bronchiolitis with wheezing may have an immunological imbalance that is expressed at the time of delivery by a lower concentration of serum IL-12. Pediatr Pulmonol. 1999; 28:175–180. © 1999 Wiley-Liss, Inc.