The objectives of this study were to measure the recruitment, to study characteristics associated with recruitment, and to explore reasons for non-recruitment in clinical trials for malignant hematological diseases. Trials opened between 2002 and 2008 were selected. If the patient fulfilled the main criteria of the protocol, all eligibility criteria of the protocol were assessed. A total of 1394 patients-protocol were identified in 17 protocols (697 patients, since a patient could have been eligible for more than one protocol) and 195 patients-protocol (186 patients) of these fulfilled the main criteria of the protocol. Among the 195 patients-protocol, 133 (68·2%) fulfilled all the eligibility criteria and 45 (23·1%) were recruited. Patients, physicians, and protocol characteristics were not associated with recruitment. The most common reasons for not being recruited were as follow: 40·7%, not fulfilling all eligibility criteria; 31·3%, protocol not being proposed according to the chart; and 22·7%, patients' refusal.