A 41-year-old patient with dermatitis herpetiformis (DH) developed steroid-resistant blebs as a sign of exacerbating DH. The skin symptoms were resolved after 2 weeks of oral administration of diaminodiphenyl sulphone (DDS). However, 3 weeks after the start of DDS, he suffered from edematous eruption on the cheeks and neck, enlargement of the pharynx, systemic lymphoadenopathy and hepatomegaly. In addition, his leukocyte count increased rapidly from 10.1 x 10(9)/l with 13% monocytes just before the start of DDS, to 24.6 x 10(9)/l with 28% monocytes. Bone marrow aspirate showed trilineage dysplasia and chronic myelomonocytic leukemia (CMML) was diagnosed. The patient died from septic shock during neutropenia following cytotoxic chemotherapy. In this case, CMML was complicated with DH and the administration of DDS accelerated the progression of CMML with the manifestations of DDS syndrome. Although DDS is a well-established drug for DH, DDS should be used with great caution when a hematological malignancy coexists.