A linear envelope assumption and quasi-resonant four-wave interactions for deep-water random waves may influence the statistical properties of deep-water surface gravity waves. As a consequence, the exceedance probability of wave height substantially deviates from the expected distribution obtained by assuming linear narrow banded spectrum waves. Here the occurrence probability of maximum wave heights is compared with ocean wave measurements during the yearsd 2001-2005 around Japan. The maximum wave height distribution and probability of occurrence of freak waves are compared with theoretical expectations and are adjusted by the measured data. The proposed distribution well predicts extreme waves in nonlinear random wave fields.