Serum neopterin may be considered a diagnostic marker of the degree of activation of the immune system. This preliminary study was performed to measure serum neopterin levels in patients with acute spontaneous urticaria (ASU), chronic spontaneous urticaria (CSU) and atopic dermatitis (AD). In total, 180 patients [96 men, 84 women; mean age 23.1 years; 41 with spontaneous urticaria (13 ASU and 28 CSU), 48 with AD] and 96 healthy controls were enrolled in the study. Patients with ASU had the highest neopterin levels, and all three groups of patients (ASU, CSU and AD) had higher values than normal subjects. This preliminary study demonstrates that serum neopterin could be a biomarker of immune activation in patients with SU or AD.