The joint distribution of wave periods and heights is investigated for 89 surface wave records at the coastal waters, and compared with the theory by Longuet-Higgins based on the assumtion of a narrow band spectrum. The observed records are classified into five groups according to the correlation coefficient between individual wave heights and periods. Correlation tables between wave periods and heights normalized with the mean values are made for five groups separately.
　The applicability of the theory is confirmed for a narrow band spectrum with the band width parameter below 0.1 by analysis of numerically simulated wave profiles. Sea waves also show agreement with the theory when the correlation coefficient between wave heights and periods is low. Disagreement appears and enlarges as the correlation coefficient increases. Even so, the portion of large wave heights in the joint distribution is well approximated with the theory on the condition that the spectral band-width parameter is to be estimated by the interquartile range of the marginal distribution of wave periods.