Mastocytosis is often associated with organ involvement and hematological disorders. Patients may also exhibit elevated levels of plasma IL-6. To gain insight into the relevance of this observation, we correlated plasma levels of IL-6 and soluble IL-6 receptor (sIL-6R) with multiple disease parameters in 29 patients with mastocytosis. Mean plasma IL-6 levels were elevated in patients compared to healthy controls (P < 0.0001). Disease category significantly correlated with plasma IL-6 levels, as did severity of bone marrow pathology, organomegaly, and extent of skin involvement. In plasma, there was a positive correlation of IL-6 to total tryptase, alkaline phosphatase, IgM, white blood cell count, prothrombin time, partial thromboplastin time, and neutrophil numbers. There was an inverse correlation to hemoglobin. sIL-6R levels were not elevated. These observations demonstrate that IL-6 is a useful surrogate marker of severity of hematologic disease and suggest that IL-6 contributes to pathology.