The aim of the present study (N = 69) was to examine whether middle childhood attachment, measured using the Separation Anxiety Test (Slough, Goyette & Greenberg, 1988), predicts aspects of social functioning (social initiative, prosocial orientation, social anxiety, loneliness) in young adulthood. Insecurity-avoidance at age 8.5 years was, as expected, negatively related to social initiative and prosocial orientation, and was also positively related to social anxiety and loneliness at age 21 years. In addition, insecurity-avoidance contributed to developmental change in social anxiety from middle childhood to young adulthood. Contrary to our expectations, the two security scales were generally unrelated to future social functioning. Taken together, these results extend previous research by showing that insecurity-avoidance is related to social functioning also beyond childhood and adolescence, and that it contributes to developmental change in social functioning over time. The scarcity of prospective links for the attachment security scales points to the need for future studies addressing when and why attachment does not contribute to future social functioning.