Many children with recurrent wheezing in early childhood develop asthma. Objective parameters to describe different groups of wheezers are limited, but tidal flow volume (TFV) response to inhaled salbutamol has demonstrated differences between children with and without asthma. Also, eosinophil cationic protein (ECP) has been associated with declining lung function in older children. We therefore investigated whether lung function and serum ECP (s-ECP) could differentiate between groups of wheezy young children. TFV loops were measured in 79 awake children (mean age 14 months). Minimum two wheezy episodes (mean 3.2) or minimum 4 weeks persistent wheeze were reported in 41 children (cases), whereas the 38 controls had no history of wheeze. Airways responsiveness (change in ratio of time until peak expiratory flow to total expiratory time (tPEF/tE) after inhaled nebulized salbutamol) was measured in 26 cases and 24 controls. Serum ECP and serum myeloperoxidase (s-MPO) were measured in all children. Cases had significantly lower mean tPEF/tE (0.21) than controls (0.33), and higher mean s-ECP (21.9 micrograms.L-1) than controls (14.0 micrograms.L-1). Serum ECP (but not s-MPO) correlated significantly with the percentage change in tPEF/tE from baseline (r = 0.7), but not with initial tPEF/tE. Serum ECP increased significantly with increasing immunoglobulin E (IgE), airways responsiveness and eosinophil count, but decreased with increasing age. TFV responsiveness to salbutamol and s-ECP levels correlate strongly, both probably reflecting airways inflammation, and may possibly be valuable prognostic tools in recurrent wheezy infants and toddlers.