Exposure to carbon black has been linked to risk of lung and bladder cancer. We therefore investigated the frequency of these cancers in a group of 2286 longshoremen who were exposed occupationally to carbon-black dust. We identified 208 cancers (standardised incidence ratio 96, 95% CI 83-109), 53 lung cancers (108, 81-141), and 32 bladder cancers (130, 89-184). Longshoremen exposed to high concentrations of carbon black (n=14) had a significantly increased frequency of bladder cancer (204, 112-343). We conclude that the increase in bladder cancer in longshoremen is probably related to high exposure to carbon black.