A 39-years-old man afferred to our hospital for a fever lasting for more than 6 months, without abnormalities at physical examination (in particular no skin alterations); a recent laboratory and instrumental investigation was ineffective and so a fever of unknown origin (FUO) was diagnosed Since he reported an history of infantile mastocytosis (usually auto-resolving) we evaluated his serum-tryptase levels that resulted of 49 ug/L (normal value 20 ug/L), raising the doubt of the presence of an active mastocytosis. The following bone marrow evaluation showed aggregates of CD117 positive cells and a c-Kit point mutation at codon D 816V confirming the diagnosis of indolent mastocytosis.The present case confirm that FUO can be caused by an otherwise asymptomatic indolent mastocytosis, thus suggesting to include the serum-tryptase level measurement in the diagnostic approach to this pathological condition, at least in selected cases.