The prognostic significance of DNA aneuploidy was studied restrospectively in 177 Stage I cutaneous melanomas. DNA content was determined by flow cytometry of propidium iodide-stained nuclei recovered from formalin-fixed, paraffin-embedded material. Of 162 evaluable histograms, 124 were diploid, 35 aneuploid, and 3 tetraploid. Aneuploidy strongly correlated with established predictors of unfavorable prognosis, namely, thickness p less than .005, level p less than 0.005, ulceration p less than 0.005, and presence of vertical growth phase p less than 0.02. Overall, aneuploidy was strongly correlated with recurrence (p less than 0.005) and shorter disease-free survival (p less than 0.0001). Aneuploidy was an independent predictor of recurrence for tumors less than 1.5 mm thick (p less than 0.0001) and greater than or equal to 3 mm thick (p = 0.031). For melanomas 1.5-2.9 mm thick, aneuploid tumors had a 27% higher recurrence rate than diploid tumors (63% vs. 36%). This was not statistically significant (p = 0.247). In a multivariate analysis of common predictors stratified by thickness, DNA aneuploidy was the most significant independent parameter (p less than 0.002). DNA content appears to be an important stratification parameter for Stage I cutaneous melanoma.