The discovery and cloning of the cyclin-dependent kinases (cdks), main regulators of cell cycle progression, allowed several investigators to design novel modulators of cdk activity. Flavopiridol (HMR 1275, L86-8275), a flavonoid derived from an indigenous plant from India, demonstrated potent and specific in vitro inhibition of all cdks tested (cdks 1, 2, 4 and 7) with clear block in cell cycle progression at the G1/S and G2/M boundaries. Moreover, preclinical studies demonstrated the capacity of flavopiridol to induce programmed cell death, promote differentiation, inhibit angiogenic processes and modulate transcriptional events. The relationship between the latter effects and cdk inhibition is still unclear. Initial testing in early clinical human trials with infusional flavopiridol showed activity in some patients with non-Hodgkin's lymphoma, renal, prostate, colon and gastric carcinomas. Main side effects were secretory diarrhea and a pro-inflammatory syndrome associated with hypotension. Biologically active plasma concentrations of flavopiridol (approximately 300-500 nM) are easily achievable in patients receiving infusional flavopiridol. Phase 2 trials with infusional flavopiridol in several tumor types, other schedules and combination with standard chemotherapies are being assessed. In conclusion, flavopiridol is the first cdk inhibitor to be tested in clinical trials. Although important questions remain to be answered, this positive experience will stimulate the development of novel cdk modulators for cancer therapy.