Objective To study the prognosis of infants with bronchiolitis by testing urine leukotriene E4 (LTE4) level and investigating atopy's influences. Methods Urine LTE4 was tested in 38 eases with mild bronchiolitis (47 in acute stage, 17 in convalescent stage), 9 severe bronchiolitis cases, 15 atopic cases, 25 control cases. Peripheral blood was used to determine eosinophils count (EC) in acute bronchiolitis cases. Results (1) The level of urine LTE4 is obviously higher in cases of acute group (62.11 ± 12.23 pmol/L) than that of control group (22.19±1.50 pmol/L) , and the convalescent group (34.86 ±5.75 pmol/L) (F = 132.42, P < 0.01) ;Urine LTE4 level of convalescent group is higher than that of the control group (P < 0.01). (2) Urine LTE4 level is significantly higher in severe group (98.04 ± 8.04 pmol/L) than that of mild group (59.16 ± 12.25 pmol/L) (t = 9.92, P < 0.01). (3) Urine LTE4 level of atopy positive (88.75 ± 10.45 pmol/L) infants with bronchiolitis is significantly higher than atopy negative infants (55.28 ± 11.44 pmol/L)(t = 8.63, P < 0.01). (4) There is no significant correlation between the levels of urine LTE4 and EC for acute bronchiolitis. Condusions The level of urine LTE4 in acute bronehiolitis patients increases and remains high in convalescent stage;Higher urine LTE4 level in severe bronchiolitis cases indicates that urine LTE4 level is related to the severity of the disease;cysteinylleukotrenes is an important mediator of inflammation that may influence the prognosis of atopy positive infants with bronchiolitis;EC is not a good index to present the airway inflammation of infants with bronehiolitis.