The aim of the introduced study was to investigate, if the altered NK-cell functions in obese individuals are a possible cause for increased cancer incidence in obesity.To induce obesity,64 male F344 rats received a high fat diet or a control diet for 6 - 10 weeks. Thereafter, animals received 106 cells of an adenocarcinoma syngeneic tumor (MADB106) or a vehicle by i.v.injection. 15 min after injection, 32 rats were killed, lungs removed and immunohistochemically stained. Afterwards, numbers of NK-, MADB106-cells and NK-cell-tumor-cell-interactions were quantified. Three weeks after tumor-cell injection the second group of rats (n=32) was killed and lung metastases were counted. After short term MADB106-challenge, obese animals showed significantly decreased NK-cell-numbers and NK-cell-tumor-cell-interactions in the lung as compared to their control littermates. Three weeks after injection, the lungs of the obese rats showed significantly more lung metastases than the control animals. Induction of obesity in F344 rats leads to altered NK-cell functions against tumor cells and results in significantly enhanced lung metastasis as compared to lean animals.