Ageing errors can introduce considerable bias in the time series of recruitment estimated from a sequential population analysis. Simulation results indicate that mild ageing errors can halve estimates of the variance of recruitment and can introduce potentially important autocorrelations in recruitment time series. Underageing resulting from the use of unvalidated ageing techniques can affect recruitment estimates sufficiently to severely hinder studies of environmental influences on recruitment. As an additional sensitivity analysis, researchers should consider recalculating recruitment after correcting catch data for ageing errors.