The distinction between the keratoacanthoma (KA) and the squamous cell carcinoma (SCC) can sometimes be difficult on the basis of histologic and clinical criteria. The possible diagnostic significance of DNA ploidy initiated the present study evaluating the DNA ploidy in paraffin-embedded tissue sections of 7 KA and 15 SCC, and fresh frozen tissue touch preparations of 15 of the same cases using the CAS 200 Image Analyzer. In paraffin-embedded tissue sections the main peak DNA index was based on normal epidermis, and ranged from 1.03 to 1.59 in KA and from 1.47-2.71 in SCC. The DNA Index (DI) discriminated KA from SCC in 17 of 22 cases (p less than 0.0007). The highest DNA content of single nuclei ranged from 9.0-18.0 picograms (pg) (DI 2.9-6.03) in KA and 14.8-38.6 pg (DI 4.0-11.03) in SCC. The highest DNA content discriminated KA from SCC in 16 of 22 cases (p less than 0.003). In fresh frozen tissue touch preparations from 15 of the same lesions, there was considerable overlap in DNA indices of KA (0.534-1.39) and SCC (0.464-1.41). Abnormal DNA peaks seen in histograms from three SCC in paraffin-embedded tissue sections were lost in the touch preparation histograms, probably due to inadequate sampling. Therefore, image analysis of paraffin-embedded tissue sections is better able to distinguish KA from SCC than touch preparations.