A 57-year-old woman was diagnosed as having monoclonal IgG kappa gammopathy of undetermined significance with Sjögren syndrome. Five years later, she was admitted with an increased level of serum IgG and diagnosed as having multiple myeloma. After admission, fever and painful erythema developed. Combined chemotherapy with adrenal cortical steroid diminished the skin lesions. Erythema recurred during treatment with granulocyte colony-stimulating factor for neutropenia due to chemotherapy. A biopsy specimen from the skin revealed dense neutrophilic infiltration in the dermis, and a diagnosis of Sweet disease was made.