A single-agent dose-escalating phase I study on the farnesyl transferase inhibitor SCH 66336 was performed to determine the safety profile and recommended dose for phase II studies. Plasma pharmacokinetics were determined as well as the SCH 66336-induced inhibition of farnesyl protein transferase in vivo. SCH 66336 was given orally once daily (OD) without interruption to patients with histologically-confirmed solid tumours. Routine antiemetics were not prescribed. 12 patients were enrolled into the study. Dose levels studied were 300 mg (6 patients) and 400 mg (6 patients) OD. Pharmacokinetic sampling was performed on days 1 and 15. Although at 400 mg OD only 1 patient had a grade 3 diarrhoea, 3 out of 6 patients interrupted treatment early due to a combination of various grade 1-3 toxicities (diarrhoea, uremiacreatinine, asthenia, vomiting, weight loss) indicating that this dose was not tolerable for a prolonged period of time. At 300 mg OD, the same pattern of toxicities was observed, but all were grade 1-2. Therefore, this dose can be recommended for phase II studies. Pharmacokinetic analysis showed that peak plasma concentrations as well as the AUCs were dose-related, with increased parameters at day 15 compared with day 1, indicating some accumulation upon multiple dosing. Plasma half-life ranged from 5 to 9 h and appeared to increase with increasing dose. Steady state plasma concentrations were attained by day 14. A large volume of distribution at steady state suggested extensive distribution outside the plasma compartment. There is evidence of inhibition of protein prenylation in some patients after OD oral administration of SCH 66336. SCH 66336 can be safely administered using a continuous oral OD dosing regimen. The recommended dose for phase II studies using this regimen is 300 mg OD.