Measurement of adherence: Adherence to lenalidomide treatment will be measured continuously throughout the study period, by several means: 1) Electronic monitoring (EM) will be used throughout the first 5 months of the study period. This is considered the gold standard for measuring adherence, reflecting adherence more objectively than questionnaires and pill-counting. The microelectronic monitoring system (MEMS), a specific type of EM, consists of an electronic device ﬁtted in the cap of a normal-looking medication bottle that automatically records each time the bottle is opened. Adherence measured with this device was associated with treatment response in the landmark study in CML by Marin et al. 2) Pill counting for first five months. 3) Questionnaires: The physician visual analogue scale (physician VAS) and an adaptation of the "Basel assessment of adherence with the Immunosuppressive Regimen Scale" (BAASIS), will be assessed at predefined time points throughout the 18 months of the study period.