We examined the extent to which attachment insecurity was related to eating disorder (ED) symptoms, and predictive of treatment outcomes. Women diagnosed with anorexia nervosa (AN) restricting subtype (ANR), AN binge purge subtype (ANB), or bulimia nervosa (BN) completed an attachment scale pretreatment, and ED symptom scales pretreatment (N = 243) and post-treatment (N = 157). A comparison sample of 126 non-ED women completed attachment scales on 1 occasion. Those with EDs had significantly higher attachment insecurity than non-ED. ANB was associated with higher attachment avoidance compared with ANR and BN, and higher attachment anxiety compared with BN. Higher attachment anxiety was significantly related to greater ED symptom severity and poorer treatment outcome across all EDs even after controlling for ED diagnosis. Attachment dimensions substantially contribute to our understanding of ED symptoms and treatment outcome. Addressing attachment insecurity when treating those with EDs may improve treatment outcomes.