A rapidly growing number of oral anticancer agents has become available in oncology and hematology. Though these introductions have several benefits, medication adherence is an issue of concern. Little is known about the factors influencing adherence to treatment with oral anticancer agents in daily practice. Material and methods. In this observational, multicenter study including 216 patients, carried out between October 2010 and March 2012, the use of oral anticancer drugs was assessed by means of a telephonic pill count, a questionnaire and a review of the patient's medical file and pharmacy medication records. Parameters collected were patients' demographics, treatment characteristics, beliefs and attitude towards disease and medicines, self-reported adherence, side effects, quality of life and satisfaction about information. Patients off treatment filled out a questionnaire about the reasons for discontinuation. Optimal adherence was defined as ≥ 95%-≤ 105%. Results. The mean adherence rate (AR) (n = 177) was 99.1% with 20.3% of patients having a sub-optimal AR (< 95%, > 105%) consisting both of under- and over-adherence. Multivariate analyses showed that being on a cyclic dosing regimen (rather than a continuous regimen), not living alone and being highly educated increased the chances of optimal adherence (ORs = 4.88, 4.59 and 2.53, respectively). In addition, optimal adherence was found to be less common in patients reporting treatment control (OR = 0.77). One third of 79 patients off treatment reported their experienced side effects as one of the reasons for discontinuation. Discussion. Although most patients are fully adherent to oral anticancer agents, there is a substantial number tending to non-adherence. Patients living alone and those on a continuous dosing regimen are most likely to adhere sub-optimally. Interventions to improve adherence should specifically address these patients and be tailored to the needs of the individual patient.