Factors that affect the response of multiple myeloma patients to thalidomide were evaluated in 40 patients who were not eligible for chemotherapy (untreated: 14, relapse/refractory: 26). The complete response (CR) rate was 2.5%; partial response (PR) 50.0%; minimal response (MR) 25.0%; no change (NC) 12.5%; and progressive disease 10.0%. The response to thalidomide could be evaluated after four weeks of treatment. Significantly higher responses were associated with untreated patients, patients with combined use of thalidomide plus dexamethasone, and patients with kappa light chain. Patients who responded well to thalidomide showed a significantly higher progression-free survival (PFS) rate. In patients with kappa light chain, PFS and overall survival rates were significantly higher than those with lambda light chain. Frequent adverse reactions were numbness (47.5%), constipation (32.5%), and eruption (30.0%). In patients previously treated with vincristine, numbness occurred in a significantly higher percentage of patients.