Melt flow data, obtained from capillaries having constant length‐to‐diameter ratios, but different diameters, do not lie on a single curve. At constant shear rate the apparent viscosity decreases with capillary diameter. This diameter effect has been interpreted by some investigators as an indication of slippage along the capillary wall. This paper presents data which demonstrate that, if corrections are made for pressure drop in the rheometer barrel, the flow curves coincide. Although it has been recognized that pressure losses in the barrel can have a significant influence on melt index values, it has usually been assumed that they are negligible in high shear measurements. However, if reliable flow curves are to be obtained, the barrel losses cannot be ignored. Furthermore, the value of losses can be used to calculate the shear stress in the barrel and thus extend the range of the measurements.