A number of immunological abnormalities have been reported in sarcoidosis. In this paper an excess of Natural killer (NK) cells, as defined by the reactivity with HNK-1 monoclonal antibody, is demonstrated in peripheral blood of these patients. On the contrary, only a few HNK-1+ cells have been found among mononuclear cells infiltrating and/or surrounding sarcoid granulomas in the lungs, lymph nodes and skin. While NK cell activity may represent one of the first lines of natural resistance against foreign antigens, the lack of killing in involved tissue gives no support for control of sarcoidosis by NK cells at sites of disease activity. Possible interpretations of these findings are discussed.