To address debate about risk associated with isolated behaviors during middle childhood, the present study utilized an extreme group approach to examine behavioral characteristics and social functioning of a large sample of children in grades 2-5 who scored 1.5 standard deviations above the classroom mean on an extensively studied, psychometrically sound measure of isolated behavior: the Sensitive-Isolated scale of the Revised Class Play (RCP; Masten, Morison, & Pellegrini, 1985). Children viewed by peers as extremely sensitive-isolated (SI) were first compared to a non-isolated comparison group (COMP) matched one-to-one on classroom, race, and gender, with regard to risk for peer rejection and friendlessness. Risk and/or protective benefits conferred by specific demographic factors (gender, race, grade-level) and behavioral characteristics (academic and athletic competencies) were examined. Regression analyses revealed that SI children were at significantly greater risk for friendlessness and peer rejection relative to COMP peers. There were no main or interactive effects of demographic variables. Main effects of poor academic and athletic abilities were shown for peer rejection and friendlessness; poorer abilities were associated with increased risk for these outcomes. No interactive effects of academic or athletic abilities with group membership were demonstrated. Latent class analyses within the SI group utilizing behavioral data from the RCP revealed the presence of three distinct classes of SI children: SI-Pure (66%), SI-Aggressive (26%), and SI-Prosocial (8%). With regard to relative vulnerability for friendlessness, the SI-Pure class did not demonstrate greater risk for friendlessness relative to the SI-Prosocial class and was less likely than the SI-Aggressive class to be friendless. The SI-Pure class was more likely to be rejected than the SI-Prosocial class. The SI-Prosocial class showed the lowest risk for peer rejection. However, this protective effect was not present for the friendlessness variable. The SI-Aggressive class evidenced significantly relatively greatest risk for friendlessness and peer rejection. Given increased risk for peer rejection and friendlessness associated with SI behaviors in middle childhood, the current study adds more evidence to the literature describing psychosocial difficulties for isolated children, particularly when these behaviors include comorbid aggression, underscoring the need for timely identification and intervention.