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Purpose: Flavopiridol potently enhances the effect of irinotecan with cures in colorectal cancer xenografts, and is associated with modulation of several molecular targets, including p21, Differentiation-related gene 1 (Drg1), and p53. We initiated a phase I trial of the sequential combination of irinotecan followed by flavopiridol to determine the maximal tolerated dose of this combination therapy. Patients and Methods: Forty-five patients with advanced solid tumors were enrolled. Irinotecan was administered first (100 or 125 mg/m2) followed 7 hours later by escalating flavopiridol (10-70 mg/m2) given weekly over 1 hour for 4 of 6 weeks. At the maximal tolerated dose, the pharmacokinetic analysis was expanded and pre- and posttreatment tumor biopsies were done. Results: At irinotecan 100 mg/m2, dose-limiting diarrhea and myelosuppression were observed with flavopiridol 70 mg/m2. At irinotecan 125 mg/m2, we observed dose-limiting hyperbilirubinemia, fatigue, and myelosuppression at flavopiridol 60 mg/m2. Peak flavopiridol concentrations of ≥2 μmol/L were achieved above flavopiridol 50 mg/m2. No significant pharmacokinetic interactions with irinotecan were noted. Baseline serum bilirubin significantly predicted cycle 1 dose-limiting toxicity and neutropenia. We observed partial responses in three patients and prolonged stable disease (i.e., >6 months) in 36% of patients including adrenocortical cancer and hepatocellular cancer. Patients with wild-type p53 and either no change or low posttreatment biopsy p21 and a decrease in Drg1 expression showed stable or responsive disease to the combination therapy. Conclusions: The recommended phase II dose with irinotecan 100 mg/m2 is flavopiridol 60 mg/m2 and with irinotecan 125 mg/m2 is flavopiridol 50 mg/m2. Toxicity can be predicted by baseline bilirubin. Clinical activity is encouraging and may correlate to changes in p21 and Drg1 levels in patients with wild type p53 tumors following therapy.