PRIMARY OBJECTIVES: I. To determine the maximum tolerated dose (MTD) and recommended phase II dose (RP2D) of leflunomide, when given as a single agent. (Phase I) II. To assess the safety and tolerability of leflunomide at each dose level by evaluation of toxicities including: type, frequency, severity, attribution, time course and duration. (Phase I) III. To evaluate the anti-myeloma activity of leflunomide, when given as a single agent, as assessed by overall response rate (ORR). (Phase II)