Purpose : To better understand the mechanisms underlying (in)sensitivity of tumors to anticancer drugs, assessing intra-tumor drug pharmacokinetics (PKs) could be important. We explored the feasibility of microdialysis in tumor tissue for multiple days in a clinical setting, using carboplatin as model drug. ; Methods : Plasma and microdialysate samples from tumor and adipose normal tissues were collected up to 47 h after dosing in eight carboplatin-treated patients with an accessible (sub)cutaneous tumor. ; Results : Pharmacokinetics were evaluable in tumor tissue in 6/8 patients and in adipose normal tissue in 3/8 patients. Concentration–time curves of unbound platinum in both the tissues followed the pattern of the curves in plasma, with exposure ratios of tissue versus plasma ranging from 0.64 to 1.46. ; Conclusions : Microdialysis can be successfully employed in ambulant patients for multiple days, which enables one to study tissue PK of anticancer drugs in normal and malignant tissues in more detail.