Opium use is one of the most common forms of substance abuse in Iran and there are some evidence indicating it is a risk factor of transitional cell carcinoma (TCC) of the urinary bladder. The majority of opium users are also cigarette smokers, so consideration of the high prevalence of smoking which is the most important risk factor of TCC of the urinary bladder among opium users is essential to assess the role of opium use as a possible risk factor of TCC. This study was done to evaluate the role of opium as a risk factor of TCC. A case-control study was performed on 255 individuals diagnosed with TCC of the urinary bladder by pathologic light microscopic examination of the tumor biopsies. Control population was chosen from individuals who had no history or presenting signs or symptoms of urinary problems. Case and control groups were matched by sex and age and also by cigarette smoking habits. Forty-one (18.1%) of the cases and 12 (5%) of controls were recognized to be opium users. Mantel-Haenszel analysis showed an odds ratio of 3.88, with 95% confidence interval of 1.99-7.57 and P value of < 0.001. Results indicate that opium use is a risk factor for TCC. The majority of opium users are also cigarette smokers, which is another important risk factor for TCC. Routine urine cytology and early evaluation in the patients presenting with any of the symptoms of urinary bladder malignancy by means of cystoscopy and urine cytology are highly recommended.