The study is designed as a prospective, non-interventional, observational single arm study. Hundred-fifty patients over 18 with stable disease or at least partial response while on treatment with lenalidomide will be recruited from approximately 20 haematology/oncology sites in the Netherlands. Recruitment will continue until 150 patients have started the study. When this target is reached, all patients on lenalidomide will continue to be followed until the last patient has been followed for 36 months.

Inclusion Criteria:

• Patients over 18 years of age who understand and voluntarily sign an informed consent form.
• Patients with multiple myeloma who are currently receiving lenalidomide treatment and who have taken lenalidomide for > 2 cycles and < 6 cycles.
• Patients with currently stable disease or meeting the criteria for complete response, very good partial response or partial response while on lenalidomide treatment.

Exclusion Criteria:

• Refusal to participate in the study.
• Women who are pregnant or breast-feeding.
• Women of childbearing potential unless all of the conditions of the Pregnancy Prevention Programme are met.
• Hypersensitivity to the active substance or to any of the excipients.

Patients already successfully, i.e. with currently stable disease or meeting the criteria for complete response, very good partial response or partial response, on-treatment for maximally up to 6 cycles with lenalidomide at each site will be enrolled into the study. In all cases, the decision to treat the patient with lenalidomide was already made prior to the decision to enter the subject into the study.