Different functional relationships for the distribution of the peak period Tp for a given seastate Hs are tested against hindcasted and measured time series for various locations in the European seas, namely Barents Sea, Baltic Sea and Western Mediterranean (Tyrrhenian) Sea. The aim is to investigate their performance when extrapolating from available data for the estimation of the long return period seastates which are not covered by the length of the time series. A second complementary objective of the paper is to quantify the uncertainty and the variability that can occur in the estimation of the Hs/Tp association when a short time series is available. This is done by assessing the Hs/Tp relation from a 35-years long time series that will be considered as the correct one. Then it will be compared with those obtained by analysing multiple cases of shorter and shorter portions of the time series. The spread of the results against the one obtained with the entire time series will be estimated to give some indications about how long a time series has to be to ensure a correct extrapolation to the long return period seastates and to provide an indication of the error that can affect the results when a short time series is available. Copyright © 2011 by ASME.