Guided by the theoretical concept of uncertainty, the present study was an exploratory investigation of general partner and relational uncertainty within sibling relationships. Undergraduate and graduate student participants responded to a written survey utilizing scales measuring general partner uncertainty, relational uncertainty, extent of topic avoidance, and various relational and demographic items. Results revealed that siblings experience significantly more partner uncertainty than relational uncertainty, although both uncertainty types were experienced at low to moderate levels. Further, differing levels of partner and relational uncertainty were associated with frequency of sibling contact, but not with sibling dyadic gender composition. Finally, both general partner and relational uncertainty were significant predictors of levels of general topic avoidance, although neither uncertainty type predicted romantic=sexual relationship topic avoidance levels. Theoretical implications for uncertainty management are discussed in light of the unique sibling relationship.