A CASE SAR analysis was performed on a selected database of PAH's to investigate the possible use of the CASE method as an aid for preliminary assessment of carcinogenic potential of untested environmental PAH's. A data set consisting of 78 PAH's and their experimental carcinogenicities was used to 'train' the CASE fragment. These fragments predicted the activities of 94% of the 'training' set correctly. Using these fragments, the potential activities of a database of 106, mostly untested PAH's were predicted and compared to 'expert judgement' predictions in order to evaluate the extent of concordance between these two methods. Initial poor agreement (64%) was attributed to inadequate CASE knowledge of 2- and 3-ring PAH subclasses; when these subclasses were excluded, the concordance improved to 90%. The prediction accuracy of 75%, despite the structural diversity of the data set, provided independent evidence of the utility of the present CASE results. A close examination of the CASE incorrect predictions was conducted to delineate inadequacies of these CASE results in order to provide cautionary guidance for future application of the method. Finally, the present results were compared to the results of a previous CASE analysis based on a more limited PAH data set, and were found to be of greater general utility.