Up to now the question remained open, whether or not the pesticide Dichlorvos, should be classified as a carcinogen. We report on our results of long-time oral administration to male and female C57B1/6/Bln-mice of Dichlorvos to test the compound for carcinogenic activity. Dichlorvos significantly increased the incidence of focal hyperplasias (transitional cell hyperplasias) of the urinary bladder in male and female mice and decreased significantly the incidence of mixed lymphomas in mice of both sexes as compared with control animals (treated with solvent or untreated). There were no neoplastic lesions found including papillomas of the urinary bladder which could be attributed to the treatment of the animals with Dichlorvos.