The objective of this study was to determine the proportion of ecologic studies published during a 20-year period regarding environmental exposures and cancer in which correlation coefficients or coefficients of determination were used as a measure of association. The authors performed a descriptive analysis of published literature by conducting a systematic review of PubMed to identify eligible ecologic studies published between 1991 and 2010. The reported measure of association was extracted for all eligible studies. During the 20-year study period, 35/105 (33%, 95% confidence limits [CL]: 25%, 43%) ecologic studies used correlation coefficients or coefficients of determination as a measure of association. These results indicate that the use of correlation coefficients and coefficients of determination as measures of association in ecologic studies of environmental exposures and cancer is relatively common, despite extensive literature discouraging their interpretation as valid measures of association.