Aim: To assess malocclusions susceptible to orthodontic prophylaxis by using the twin-method of analysis. Material and Methods: A comparative analysis of malocclusion was performed in monozygotic and dizygotic twins (164 twin-pairs: 90 monozygotic pairs composed of 50 male and 40 female twin-pairs, and 74 dizygotic twin-pairs, with 46 male twin-pairs and 28 female twin-pairs). The zygotic twin-status certainty was 98%, assigned using serologic and morphologic criteria together with dermatoglyphs. The Allen equation was applied for calculation of the hereditary coefficient for a given malocclusion. Significance established at P < .05 was used to analyze the data. Results: Different distribution of within-pair malocclusions depended upon the gender of the individuals; nevertheless, the differences with statistical significance occurred only in Class II males and females and males with crossbites. High frequency of within-pair malocclusion similarity (80% to 94%) characterized all groups except the dizygotic males, where the magnitude was 2 times less. The differing distribution of the dentoalveolar discrepancies was not statistically significant (P > .05). The frequency of within-pair similarity equaled 100% in monozygotic patients, while only 57% was found in dizygotic females. The hereditary coefficient had low or negative values—only in males did the examined values exceed 10%. Conclusion: This investigation showed that environmental factors account for more malocclusions than previously believed. This result encourages early intervention to intercept, disrupt, and diminish the effects of malocclusions. World J Orthod 2007;8:405–410.