This study analyzes cross-sectional data obtained from respondents in neo-Nazi online discussion forums and textual data from postings to these forums. It assesses the impact of participation in radical and homogeneous online groups on opinion extremism and probes whether this impact depends on political dissimilarity of strong and weak offline ties. Specifically, does dissimilarity attenuate (as deliberative theorists hope) or rather exacerbate (as research on biased processing predicts) extreme opinions? As expected, extremism increases with increased online participation, likely due to the informational and normative influences operating within online groups. Supporting the deliberative and biased processing models, both like-minded and dissimilar social ties offline exacerbate extremism. Consistent with the biased processing model, dissimilar offline ties exacerbate the effects of online groups. The theoretical and practical implications are discussed.