A 37-year-old woman was diagnosed with chronic phase chronic myeloid leukemia. 
Nilotinib treatment was initiated; however, it had to be discontinued due to an 
allergic reaction one month later, and dasatinib treatment was provided. 
Although favorable response was obtained, she started complaining of shortness 
of breath 7 months after initiating dasatinib treatment. Chest X-ray and 
echocardiography indicated pulmonary congestion and hypertension. Further, she 
was diagnosed with mixed connective tissue disease (MCTD) based on Raynaud 
phenomenon, swollen fingers, sclerodactyly, pancytopenia, hypocomplementemia, 
and positive anti-U1-RNP antibody. Consequently, dasatinib treatment was 
discontinued, and she was administered prednisolone (1 mg/kg/day), which was 
effective and successfully tapered with concomitant administration of 
cyclophosphamide. This is the first case of MCTD that developed during dasatinib 
treatment. However, because the present case was a young woman, the development 
of MCTD could probably be attributed to autoimmune diatheses or it may be a 
coincidence. However, the possibility of patients receiving dasatinib treatment 
developing autoimmune diseases needs to be assessed.