Data on water treatment and the quality of the finished water supply of the 100 largest U.S. cities (population over 60 million) were combined with data on cancer deaths (age‐adjusted) of the county or counties in which the cities lie, to look for significant correlations with altitude and with quality of water supply. The previously discovered decrease in cancer rates with increasing altitude (for countries around the world) was confirmed for these cities. Correlation coefficients, ‐0.46 for males and — 0.32 for females, were very highly significant (p < 0.001). The rates were then “altitude‐adjusted” for the subsequent analysis.