An epidemiological study was conducted regarding formaldehyde exposure of 26,561 workers employed during the 1930's at 10 formaldehyde manufacturing or using plants. The study utilized mortality data dating from 1930. Historical exposures were estimated by industrial hygienists using monitoring data available from participating plants, comments from long-term workers, and comprehensive monitoring data collected for this study. Estimates were made for each unique job title, work area and calendar-year combination. Measures of exposure to formaldehyde used in the analyses included level or intensity, duration, average, cumulative, peak, and presence of particulates. Mortality from all causes combined was approximately as expected based on mortality rates of the U.S. population. Cancer, overall, was not related to formaldehyde exposure. Exposed workers had a slightly increased incidence of Hodgkin's disease and cancers of the lung and prostate, but these were not consistently correlated with average, cumulative, or peak formaldehyde exposure levels. Similarly, mortality from cancers of the nasopharynx and oropharynx was increased, but occurrences did not show a consistently rising risk with level of exposure. The study failed to show conclusive evidence that mortality from cancer is associated with formaldehyde exposure at the levels experienced by these workers.