Aims The aim of the study was to determine the effect of renal impairment and prior platinum-based chemotherapy on the toxicity and pharmacokinetics of oral topotecan and to identify recommended doses for patients with renal impairment or prior platinum-based (PB) chemotherapy. Methods A multicentre phase I toxicity and pharmacokinetic study of oral topotecan was conducted in patients with advanced solid tumours. Patients were grouped by normal renal function with limited or prior PB chemotherapy or impaired renal function (mild [creatinine clearance (CLcr)=50-79 ml min-1], moderate [CLcr=30-49 ml min-1], severe [CLcr <30 ml min-1]). Results Fifty-nine patients were evaluable. Topotecan lactone and total topotecan area under the concentration-time curve (AUC) was significantly increased in patients with moderate and severe renal impairment (109% and 174%, respectively, topotecan lactone and 148% and 298%, respectively, total topotecan). Asian patients (23 in total) had higher AUCs than non-Asian patients with the same degree of renal impairment. Thirteen dose-limiting toxicities (DLTs) were observed, which were mostly haematological. The maximum tolerated dose (MTD) was 2.3 mg m-2 day-1, given on days 1 to 5 in a 21 day cycle, for patients with prior PB chemotherapy or mild renal impairment, and 1.2 mg m-2 day-1 for patients with moderate renal impairment (suggested dose 1.9 mg m-2 day-1 for non-Asians). Due to incomplete enrolment of patients with severe renal impairment, the MTD was determined as ≥ 0.6 mg m-2 day-1 in this cohort. Conclusions Oral topotecan dose adjustments are not required in patients with prior PB chemotherapy or mildly impaired renal function, but reduced doses are required for patients with moderate or severe renal impairment.