To obtain approval from the Ministry of Health, Labor and Welfare of Japan, a phase II study was conducted to assess the pharmacokinetics and pharmacodynamics of thalidomide along with its efficacy and safety in Japanese patients with multiple myeloma. Between 2005 and 2006, 42 patients were enrolled, and 37 patients met eligibility criteria. Of the 37 patients, 3 were excluded from efficacy analysis because of short duration of thalidomide administration (<4 weeks). The overall response rate was 35.3% (12/34), including partial response of 14.7% (5/34) and minimal response of 20.6% (7/34). The adverse events observed in high frequency (>40%) were leukopenia, neutropenia, drowsiness, dry mouth, and constipation. Grade 3 neutropenia was observed in nine cases. Peripheral neuropathy and eruption were observed in about one-quarter of the patients. Deep vein thrombosis was not observed. At a single oral dose of thalidomide (100 mg), the C (max) was 1.68 +/- 0.41 microg/ml, T (max) was 4.54 +/- 1.71 h, T (1/2) was 4.86 +/- 0.44 h, and AUC was 15.87 +/- 3.05 microg h/ml. Low-dose thalidomide was an effective and tolerable treatment for Japanese patients with relapsed/refractory myeloma. Leukopenia and neutropenia were the most serious adverse events. The pharmacokinetics was similar to those observed in Caucasian patients.