A 15-year-old, woman, Crohn's disease patient, who carried the TPMT *3C 
heterozygous mutant, complained of alopecia 3 days after starting 
6-mercaptopurine (6-MP) and then developed severe myelosuppression 6 weeks after 
starting 6-MP. The alopecia involved scalp hair only (body hair preserved) and 
was dominant in the temporal region. Following these side effects, transient 
remission of Crohn's disease occurred. Myelosuppression due to 6-MP is a rare 
but life-threatening side effect that is difficult to predict despite continuous 
monitoring of complete blood cell counts. In the present case, 6-MP-induced 
alopecia preceded myelosuppression and progressed rapidly as the 
myelosuppression worsened.