The full-scale commercial appearance of antibiotics in the 1950s caused a shift in the nature of lethal diseases from infectious and acute to noninfectious and chronic. In this situation, biological response modifiers (BRMs), which are not based on selective toxicity, could be expected to be useful. Several types of BRM exist, including retinoids, which act directly on cells at the level of gene expression, and thalidomide and related molecules, which modulate the production of various cytokines. We have been engaged in medicinal, chemical, and structural development studies based on these bioactive compounds. Retinoids include all- trans-retinoic acid (ATRA), a major active form of vitamin A (retinol), and its bioisosters, which elicit their biological effects by binding to their nuclear receptors, retinoic acid receptors (RARs). ATRA has been used in differentiation therapy, typically for the treatment of acute promyelocytic leukemia, and the treatment of dermatological diseases. Our structural development studies of retinoids, including computer-assisted molecular design, have yielded class/subtype-selective agonists, synergists, and antagonists of RARs and their partner nuclear receptors, retinoid X receptors. Among them, the benzanilide-type compounds, Am80 and TAC101, are under phase II and I/II clinical studies in Japan and the USA, respectively. Thalidomide is a hypnotic/sedative drug that was withdrawn from the market because of teratogenicity. However, thalidomide has been established to be useful in the treatment of various diseases including cancer. Thalidomide elicits a wide range of pharmacological effects, including anticachexia, anti-tumor-promoting, antiangiogenic, immunosuppressing, antiviral, hypoglycemic, and antimetastatic activities. We have found that thalidomide is a multitarget drug. Hypothetical target events/molecules of thalidomide include tumor necrosis factor-alpha production, nuclear androgen receptor, cyclooxygenases, aminopeptidases, and alpha-glucosidase. Specific and potent compounds for each of these target phenomena/molecules have been prepared by appropriate modification of the thalidomide structure, and are expected to be superior lead compounds for novel immunomodulators, antiangiogenic agents, and anti-tumor-promoting agents.