Hemophagocytic syndrome is a proliferative disorder of an activated mo
nocyte-macrophage system and is characterized by fever, hepato-splenom
egaly and pancytopenia. The serum level of interferon-gamma in the syn
drome is increased but its origin is unknown, Here we describe a case
of NK cell leukemia with hemophagocytic syndrome with elevated serum l
evel of interferon-gamma. The levels of various cytokines were monitor
ed during the course and statistic analysis was performed, To identify
the clonal component, the NK cell fraction was sorted from the mononu
clear layer and was subjected to Southern blot hybridization with a pr
obe for EB virus tandem repeats. The fraction was also stimulated with
interleukin-2 and the level of interferon-gamma in the conditioned me
dium was measured. Levels of M-CSF and interferon-gamma were significa
ntly correlated with the degree of clinical manifestations and laborat
ory data. Southern blot hybridization revealed monoclonality of an NK
cell fraction. The fraction also released interferon-gamma, Since macr
ophage can be activated through cytokines, the hemophagocytosis might
have been triggered by factor(s) released from the abnormal NK cell cl
one at least in this case.