Serum levels of Interleukin (IL)-4, Interferon (IFN)-γ and soluble CD23 (sCD23) were analysed in a prospective study of 64 infants who were monitored from birth to 18 months of age. The findings were related to family history of atopy and the development of allergic disease in the infants. Low levels of IL-4 were detected in 10 of 63 cord blood samples (median 0.14 and range 0.32 μg/1). The levels then increased, both in healthy and atopic infants, reaching a peak at either 6 or 9 months and then decreased up to 18 months of age. The children who developed atopic disease during the first 18 months of life had significantly higher IL-4 median levels than those who did not, i.e. 0.24 (range 0.40) vs <0.10 μg/1 at 3 months, (P< 0.001), 0.40 (range 0.95) vs 0.13 (0.19) μg/1 at 6 months (P < 0.01), 0.46 (range 0.78) vs 0.10 (0.24) μg/1 at 9 months (P< 0.001) and 0.30 (range 1.38) vs 0.10 (0.36) μg/1 at 18 months (P< 0.001). The IFNγ levels were below the detection level, i.e. < 100 ng/1 in all but 49 of the 196 serum samples that were analysed. There was no significant relationship with clinical outcome, nor with S-IgE levels. Soluble sCD23 levels increased in the infants with age. There was no association with either atopic disease, family history of allergic disease or IgE antibody levels. In conclusion, IL-4 levels in serum, but not sCD23 and IFN7 are associated with allergic disease in infancy. Elevated levels were recorded before the onset of clinical symptoms. The findings support that atopic disease is associated with a primary abnormality of T-cell function.