Maternal smoking during pregnancy was also positively associated with bronchiolitis risk (normalised by CD45+ cells: aOR = 99.14, 95% CI 6.37 to 1543.7, P = 0.001; absolute numbers: aOR = 72.09, 95% CI 6.02 to 863.81, P = 0.001). This evidence concerns the gene PTPRC and bronchiolitis.