CD30 is a transmembrane molecule that may be expressed on a proportion of activated T-lymphocytes and has been reported to be a marker of Th2 phenotype. A soluble form of CD30 (sCD30) is released by CD30+ cells in vivo. Our objective was to evaluate serum sCD30 levels in children with atopic dermatitis (AD) or bronchial asthma and to investigate its relation to disease severity. This study included of 60 infants and children, of whom 18 had AD, 22 had bronchial asthma and 20 were healthy matched subjects. Severity of AD was assessed according to the objective Scoring Atopic Dermatitis (obj-SCORAD) index. Laboratory investigations included complete blood count, serum total immunoglobulin E (IgE) and serum sCD30 by ELISA. Serum levels of sCD30 in AD (77.7+/-27.9 U/ml) and asthmatic patients (49.2+/-21.5 U/ml) were significantly increased compared with the control group (18.2+/-7.0 U/ml) (t=8.8, p<0.0001; t=6.4, p<0.0001, respectively). In patients with AD, sCD30 levels were shown to correlate with obj-SCORAD (r=0.96, p<0.0001). Patients with moderate persistent asthma had significantly elevated sCD30 levels than those with mild persistent asthma (t=3.4, p<0.01). In addition, sCD30 was inversely correlated to peak expiratory flow rate (r=-0.78, p<0.0001). Levels of sCD30 did not correlate with age, disease duration or serum total IgE (p>0.05). In conclusion, serum sCD30 levels correlate with the severity of AD and bronchial asthma. It appears to be an additional objective marker that may be useful for follow up and may help to improve research and management of these diseases.