A gas turbine engine comprising in axial flow series: a fan having a fan rotor; a series passages defined between adjacent outlet guide vanes for guiding flow from the fan; and a bifurcation. The gas turbine engine further comprises a substantially annular fluid passageway extending from the fan to the bifurcation, the outlet guide vanes being positioned within the passageway. The passageway includes a profiled region. The profiled region includes a first circumferential portion positioned adjacent a second circumferential portion, the first circumferential portion having a first average radial thickness and a second circumferential portion having a second average radial thickness, the first average radial thickness being smaller than the second average radial thickness, wherein the first circumferential portion is positioned in a region corresponding to a circumferential position of the bifurcation, and wherein the profiled region is at an axial position corresponding to the axial position of the outlet guide vanes and the first circumferential portion extends continuously over two or more of the passages.