The sampling variability of spectra of wind-generated waves is tested against the predictions of the theory of waves as a stationary random quasi-Gaussian process. Both laboratory data, in which stationarity was prescribed, and field data, in which the external conditions were remarkably steady, were treated in the same way. It is demonstrated that the theory of stationary Gaussian processes provides accurate estimates of the sampling variability. For a record length of 17 min, commonly used in wave monitoring at sea, the uncertainties in the significant height and peak frequency estimates are approximately ¿12% and ¿5% respectively at the 90% confidence level. Furthermore, the height of the peak of the spectrum is generally overestimated.