Therapy-related myelodysplastic syndrome (t-MDS) and acute nonlymphocytic leukemia (t-ANLL) are dramatic complications of cancer chemotherapy. Drugs like plant alkaloids or antimetabolites have not been reported to cause either t-MDS or t-ANLL. Monosomy 7(-7) and trisomy 8(+8) are among the most common abnormalities in myelodysplastic syndromes. Both abnormalities in two different clones of the same patient are very rarely reported. Such a myelodysplastic syndrome occurring shortly after treatment with an antimetabolite, the adenosine analogue cladribine (1-chlorodeoxadenosine), and involved field radiotherapy is reported here.