Systemic mastocytosis may be primary or be associated with other hematological malignancies, most commonly myeloid neoplasms. In contrast, the association of systemic mastocytosis with lymphoid neoplasms is comparatively rare, and in the absence of appropriate clinical vigilance the diagnosis may be missed. We report a case of aggressive systemic mastocytosis associated with chronic lymphocytic leukemia (SM-CLL), placing particular emphasis on the investigational algorithm used in establishing this diagnosis, and discussing key clinical considerations in the initiation of therapy.