A phase I-II clinical trial was conducted to determine the maximum-tolerated dose (MTD) of oral cyclosporine (CsA) and vinblastine in patients with metastatic renal cell cancer (RCC) as well as to estimate the response rate. Sixteen patients received a 5 mg/kg oral loading dose of CsA followed by 3 days of CsA in 4 divided daily doses escalating from 10 mg/kg per day up to 17 mg/kg per day. Vinblastine (Vbl) was administered as an intravenous bolus on the morning of the 3rd day with dose escalation from 6 to 10 mg/m2. Cycles were repeated every 4 weeks until tumor progression. Forty-nine cycles of CsA with vinblastine were administered. The maximum tolerated dose of Vbl was 10 mg/m2, with neutropenia as the dose-limiting toxicity resulting in one death. CsA could not be escalated above 17 mg/kg per day because of nausea and vomiting. Other toxicities included constipation (100%), malaise (100%), temporary increase in pain (36%), and one seizure that may have been drug-related. There were no clinically significant changes in renal function or serum bilirubin. Mean peak whole-blood CsA level at the highest CsA dose level was 919 ng/ml (range: 414-1,827) with a trough prior to Vbl injection of 451 ng/ml (range: 128-1,229). There were no tumor responses. The combination of oral CsA and Vbl is not nephrotoxic but is poorly tolerated. In most patients optimal blood levels of CsA for reversal of MDR cannot be reliably achieved, and vinblastine dose intensity must be compromised because of the significant toxicity of this regimen.