Long circular cylindrical shells may locally buckle at lower loads than those predicted by classical linear Donnell solutions due to long range interaction between Euler and local buckling. In fact, Euler buckling may be viewed as a special case of local buckling in which both the longitudinal and circumferential wave numbers are unity. This paper investigates this phenomenon for composite tubes as a function of lay-up. Tubes that are reinforced in predominantly the longitudinal or circumferential directions suffer the least knockdown of approximately 10% for tubes that are designed to buckle concurrently by Euler and local mechanisms. The greatest knockdown of approximately 35% occurs for isotropic laminates. The effect of material lay-up has been taken account of by forming a non-dimensional length parameter that is a function of both tube geometry and material properties. An interaction formula is proposed to help in design situations.