Recent data suggest that carbon nanomaterials can act as antitumor agents themselves by increasing the efficiency of cytotoxic agents when applied in combination. Here, carbon nanofibers (CNFs) and multi-walled carbon nanotubes (CNTs) were investigated regarding their impact on cellular function, cellular uptake and ability to sensitize cancer cells of urological origin to the conventional chemotherapeutics cisplatin and carboplatin. CNFs and CNTs (1-200 microg/ml) showed a low to moderate impairment of cellular function with CNFs being more deleterious than CNTs. Inhibition of cellular viability by the nanomaterials was about 20% at most. In combinatory treatments, CNFs and CNTs markedly enhanced the effects of cisplatin and carboplatin on cellular viability by 1.2- to 2.8-fold in prostate, bladder and cisplatin-resistant prostate cancer cells in comparison to the individual effects of the chemotherapeutics. Particularly the cell viability-diminishing effect of CNFs alone and in combination with the chemotherapeutics was more pronounced with dispersions prepared with human serum albumin than with phospholipid-polyethylene glycol. Albumin might mediate the cellular uptake of carbon nanomaterials which was underlined by the co-localization of albumin and carbon nanomaterials along the cellular surface as evidenced by fluorescence microscopy. Transmission electron microscopy revealed that both carbon nanomaterials were internalized by cancer cells, thereby possibly leading to an enhanced accumulation of the chemotherapeutic drugs. In fact, CNFs enhanced the cellular accumulation of carboplatin by 28% as compared to the single treatment with carboplatin. In conclusion, carbon nanomaterial-based applications could present a new strategy to overcome chemoresistance by sensitizing cancer cells to conventional chemotherapeutics.