The dynamic pressures around a large diameter vertical cylinder resting on the flume bed and piercing the free surface are measured under regular wave conditions and compared with the results of MacCamy and Fuchs' linear diffraction theory. The linear diffraction theory has been modified to account for the nonlinear effects. The results mostly pertain to deep water conditions with drag effects being negligible and are presented in nondimensional form. The agreement between the theory and experiments are in general found to be good.