Separation and stall of an impulsively started elliptic cylinder at an angle of attack are investigated through the method of inner and outer expansions. The assumptions are large Reynolds numbers and small times. The interactions between the boundary layer and the outer inviscid flow are taken into consideration. Uniformly valid solutions are obtained for general two-dimensional cylinders. The movements of the separation points on an elliptic cylinder are calculated. Stall is found to be strongly dependent on the thickness ratio and the angle of attack of the cylinder but to be weakly dependent on the Reynolds number.