Evidence is presented that opium addition is a risk factor for cancer of the bladder. A case-control study of 99 bladder cancer patients admitted to Nemazee Hospital in Shiraz, Iran was evaluated. Cancer patients and controls, matched by age and sex, were analyzed as to their opium and/or cigarette smoking habits. A high correlation between opium addiction and bladder cancer has been observed. This evidence significantly exceeded the one observed in relation to cigarette smoking only. The sex ratio for this cancer site, from hospitalized cases in Southern Iran, is estimated to be about nine male cases per one female case. This high male to female ratio was attributed to the greater addiction of males to opium. It was concluded that opium and, more likely, its pyrolysis derived fractions may represent potential bladder carcinogens in man.