The objective of the present study was to assess the effect of adding specific prebiotics to standard formula feeding on the number of fever episodes in the first year of life. In the present randomised, double-blind, placebo-controlled trial in seven centres in five West European countries, 830 healthy term infants, without a first-degree family history of allergic disease, of mothers who indicated to give only formula feeding were randomised either to receive a standard non-hydrolysed cows' milk-based formula to which a mixture of specific oligosaccharides was added (prebiotics group (PG)), or to receive a similar formula without oligosaccharides (control group (CG)). A separate reference group consisted of 300 breast-fed infants. The primary outcome was the number of fever episodes prospectively documented by the parents. There was no difference in the number of fever episodes between the PG (median value 1·19; 25th-75th percentile 0·09-2·34) and CG (1·16; 25th-75th percentile 0·06-2·38). The median number of fever episodes in the separate breast-feeding reference group was 1·24 (25th-75th percentile 0·51-3·45). There was no effect of adding specific prebiotics to standard formula feeding in reducing the number of fever episodes in the present study.