BACKGROUND Beyond their role in hemostasis and thrombosis, platelets are important for modulating inflammatory reactions. Activated platelets play a role in the pathomechanism of inflammatory diseases such as asthma, but little is known about platelet activation in chronic skin inflammation, including atopic dermatitis (AD) and psoriasis. Furthermore, the relationship between platelet activation and disease severity is not understood. This work was performed to investigate plasma levels of beta-thromboglobulin (beta-TG) and platelet factor 4 (PF4) as platelet activation markers in patients with AD or psoriasis, and to determine the relationships between these markers and disease severity. METHODS Plasma levels of beta-TG and PF4 were measured by enzyme-linked immunoassay in 22 healthy controls, 44 patients with AD, and 16 patients with psoriasis. The relationships between these markers and the scoring AD (SCORAD) index, blood eosinophilia, serum IgE and serum lactate dehydrogenase were investigated in AD patients, and relationships with the psoriasis area and severity index (PASI) score were examined in psoriatic patients. RESULTS Plasma beta-TG and PF4 levels were significantly higher in patients with AD or psoriasis compared with healthy controls. beta-TG and PF4 levels correlated with the SCORAD index, and PF4 levels correlated with PASI scores. Elevated beta-TG and PF4 levels were significantly reduced after treatments. CONCLUSIONS Our results show that blood platelets are activated in patients with AD or psoriasis, suggesting that activated platelets play a role in the pathomechanism of chronic skin inflammation. Furthermore, plasma beta-TG and PF4 may be markers for the severity of AD and psoriasis.