The purpose of this study is to assess the benefit of lonafarnib (versus placebo) in patients with myelodysplastic syndrome (MDS) or chronic myelomonocytic leukemia (CMML). Benefit will be measured by achievement of platelet transfusion independence for at least 8-consecutive weeks, and without simultaneous worsening of hemoglobin and/or need for red blood cell (RBC) transfusion. Additional endpoints will be hematologic response (which includes complete remission, partial remission, hematologic improvement), number of RBC transfusions, bleeding events, infections and safety.

Inclusion Criteria:

• Confirmed MDS (RA, RARS, RAEB, RAEB-T) or CMML according to FAB classification.
• Platelet transfusion dependence (requiring 1 to 8 platelet transfusion events every 4 week period (Day 84 to Day 57, Day 56 to Day 29, and Day 28 to Day 1) over an 8-week retrospective and 4-week prospective screening period).
• The individual number of platelet transfusion events during the three 4-weekly periods (Day 84 to Day -57; Day -56 to Day 29; Day -28 to Day -1) must not differ by greater more than 2 from the average number of platelet transfusion events during the 12 week screening period.
• If the subject is RBC transfusion dependent, the number of RBC transfusion events during the three 4-weekly periods (Days -84 to -57; Day -56 to Day 29 and Day -28 to Day -1) must not differ by more than 2 from the average number of RBC transfusion events during this 12 week screening period.

ECOG PS 0-2.

Exclusion Criteria:

• Subjects with chemotherapy/radiotherapy-associated secondary MDS.
• <12 Weeks (prior to Day-1 Randomization) from any investigational drug use, any chemotherapy, radiotherapy, immunotherapy and any other treatment or MDS/CMML other than best supportive care.
• Hx of bone-marrow or peripheral stem-cell transplantation or treatment with donor lymphocyte infusion.
• Hx of AML.
• Known hx of immune thrombocytopenic purpura.
• Marked baseline prolongation of QTc interval, CTCAE Grade >=1.
• Use of ketokonazole within 72 hours prior to study drug administration.