The author responds to a critical analysis by Hogan (American Industrial Hygiene Association Journal, 54(8):458-460, 1993) of a NIOSH study that found 4,4'-methylene-bis(2-chloroaniline) (101144) (MBOCA) to be a potential human carcinogen. The original study, initiated by a request from the Michigan Department of Public Health (MDPH) to NIOSH, found bladder tumors in three former employees. Hogan's major criticism was that the exposure characterizations of the workers were inadequate and incompletely reported. Ward noted that the company did not have complete records on all the products manufactured, making it impossible to fully characterize the workers exposures; exposures to other known and suspected human carcinogens may have existed. There was no evidence that the workers had been exposed to confirmed human bladder carcinogens such as benzidine (92875), beta-naphthylamine (91598), or 4-aminobiphenyl (92671). Hogan's criticism failed to mention MDPH data on urinary excretion of MBOCA which indicated that some factory workers had MBOCA concentrations as high as 59,000 micrograms per liter in their urine. Contamination by MBOCA was so widespread throughout the factory and in the surrounding community that the area was declared a Superfund site. NIOSH did not state that MBOCA was a confirmed human bladder carcinogen, as implied by Hogan, but that it was a potential carcinogen. The author concludes that while NIOSH had only a limited assessment of chemicals at the factory other than MBOCA, Hogan's criticism does not provide a viable alternative for the three bladder tumor cases. Based on the study data and the results of three animal carcinogenesis bioassays MBOCA should be regarded as a potential occupational carcinogen.