Formaldehyde is a naturally occurring chemical found in every human cell. It has been in widespread use for over a century as a disinfectant and preservative agent, and more recently in a number of industrial products. Animal studies indicate that formaldehyde is a rat carcinogen at high levels >> or = 10 ppm) of exposure. Results for lower levels of exposure show less clear-cut carcinogenic effects, and some species, such as mice and hamsters, appear much less sensitive to any carcinogenic potential of formaldehyde. Epidemiologic studies of the effects of formaldehyde exposure among humans provide inconsistent results. In general, these nonexperimental studies suffer from a number of biases and flaws. The epidemiologic studies fall into three categories: formaldehyde industry workers, case-control studies, and studies of professionals who use formaldehyde. Studies of industry workers with known exposure to formaldehyde report little evidence of an excess cancer risk. Nasopharyngeal cancer, the one cancer considered most strongly linked to formaldehyde among humans, appears after close examination to be likely a result of multiple subgroup analyses and misclassification. The case-control studies usually lack any direct measure of formaldehyde exposure and rely instead on hypothetical exposure based on occupational exposure matrices. Most of these studies, after adjustment for confounding factors, fail to find a significant association with putative formaldehyde exposure. The studies that do report a significant association suffer from methodologic problems limiting their interpretation.(ABSTRACT TRUNCATED AT 250 WORDS)