Rosai–Dorfman Disease (RDD) or Sinus Histiocytosis with Massive Lymphadenopathy (SHML) is a rare disorder typically manifesting as enlarged lymph nodes with or without systemic involvement. These cases are often clinically misdiagnosed as lymphoma. Recognising this entity to distinguish it from lymphoma and other causes of histiocytosis is important because of different treatment modalities for these disorders. Extranodal involvement is also common, often with a particular predilection for the head and neck region. We report a rare case of Rosai–Dorfman Disease with both nodal and extranodal involvement in a 33-year-old woman. The patient had bilateral cervical lymphadenopathy and diffuse thyroid enlargement. Thyroid gland involvement in RDD diagnosed on fine needle aspiration cytology (FNAC) has rarely been reported in literature. FNAC is a useful and reliable tool for the diagnosis of RDD and the biopsy can be avoided in these patients, thus reducing inconvenience to patients. Diagn. Cytopathol. 2011; © 2011 Wiley-Liss, Inc.