Leukopenia is a blood disease in which the number of circulating white blood cells diminishes. All underlying causes of leukopenia are not yet known. The subjects of this study are 15 leukopenic patients who were assessed by a systemic workup, including physical examination, blood tests, and molecular analysis. A common and unusual cause was revealed in all patients. This cause was a disorder with a laboratory characteristic of leukocytosis, namely familial Mediterranean fever (FMF). It was discussed that leukopenia arising in the context of FMF is mainly due to autophagy and apoptosis processes. These two pathophysiological characteristics of FMF were thought to explain the particular (episodic and self-limited) leukopenia in this disorder. Based on the results of this study in conjunction with the currently existing literature data, we suggest that FMF causes leukopenia. Leukopenic cases should be investigated for FMF, particularly if the leukopenia is episodic in nature. Early recognition of FMF would help to skip unnecessary invasive procedures and to prevent the development of amyloidosis, the devastating complication of FMF.