Because nuclear facilities can release radionuclides into the surrounding environment accidentally or during normal operations, there has been public concern over the possibility of adverse health effects. Two former nuclear materials processing facilities in Armstrong County Pennsylvania have been the focus of such public concern for over 20 y. The Apollo and Parks facilities processed uranium and plutonium fuels for use in nuclear applications. To evaluate the possibility of increased cancer rates in the communities near the Apollo-Parks nuclear processing materials plants, cancer incidence rates were assessed for the years 1993-1997, or nearly 40 y after the plants had begun operation in 1957 and 1960, respectively. The rates of cancer were evaluated among the approximately 17,000 persons living in 1 of 8 municipalities encompassing or near these nuclear sites. Numbers of cancers and mailing addresses (n = 935) were obtained from the Pennsylvania Department of Health. Because mailing addresses in small rural areas do not always reflect actual residences within a municipality, each of 935 addresses was validated (and corrections made when indicated) by contacting area postmasters and using Census Bureau geocoding information, street maps, and aerial photographs. Standardized Incidence Ratios (SIRs) were computed as the ratio of observed numbers of cancers in the study area compared to the expected number derived from general population rates of Pennsylvania. Forty percent of the mailing addresses were found not to be within the boundaries of the study municipalities. After excluding these persons who did not reside in one of the eight municipalities near the Apollo-Parks facilities, 581 cancers remained in contrast to 574.0 expected (SIR 1.01; 95% confidence interval 0.93-1.10). Based upon knowledge of the tissues where uranium or plutonium likely would be deposited after intake, cancers of the lung (SIR 0.88), kidney (SIR 1.05), non-Hodgkin's lymphoma (SIR 1.10), liver (SIR 0.61), and bone (2 observed vs. 1.19 expected) were carefully evaluated, but no significant excesses were noted at these sites. Cancers of the female breast and thyroid and leukemia also were not significantly increased, as expected since these tissues are not sites where uranium or plutonium would concentrate. Overall, no increase in cancer risk could be attributed to living near the two former nuclear materials processing facilities. However, misleading elevations in cancer risks would have been suggested if mailing addresses had not been corrected to exclude addresses that were not within the boundaries of the municipalities for which population data were available. The study had sufficient power to exclude increased cancer risks of 10% or greater.