4,4′-Methylenedianiline (MDA), an aromatic amine, is a known human hepatotoxin and an animal carcinogen but there is little information regarding its chronic effects in humans. Between 1967 and 1976, 10 workers at a plant in Ontario that used MDA as an epoxy hardener developed acute jaundice. We followed this group from the date of intoxication through to the end of 1991 for cancer incidence by matching with the Ontario Cancer Registry. To date, one cancer, a pathologically confirmed bladder cancer has developed (expected number based on provincial incidence rates: 0.64 for all cancers, 0.05 for bladder cancer). This finding may be important because bladder cancer was a site of interest a priori; bladder cancers have been observed in two other occupationally exposed groups (significantly higher than expected in one of these); in the National Toxicology Program bioassay, urinary bladder tumors occurred in exposed animals but not in controls; and MDA has structural similarity to known human bladder carcinogens such as benzidine.