A 64-year-old woman with mild bilateral parotid gland swelling and bilateral lower extremity purpura was admitted for evaluation of xerostomia and pancytopenia. The patient had an increased erythrocyte sedimentation rate, pancytopenia, and positive tests for antibodies to nuclear antigen, SS-A, and SS-B. Impaired cell-mediated immunity was also present. Bone marrow aspiration showed a hypoplastic marrow with an increased percentage of lymphocytes. A positive Schirmer's test and keratoconjunctivitis were also noted. A diagnosis of primary Sjögren's syndrome was made by sialography and histological salivary gland findings. Therapy with prednisolone improved the pancytopenia. Addition of the patient's peripheral blood mononuclear cells to cultures of bone marrow mononuclear cells derived from a healthy volunteer dose dependently inhibited colony formation, including mixed hemopoietic colonies. On the other hand, addition of the patient's serum failed to inhibit colony formation by normal bone marrow mononuclear cells. These results suggested that one of the causes of pancytopenia in primary Sjögren's syndrome might be mediated by the inhibition of mononuclear cells to the hemopoietic progenitors.