BACKGROUND AND AIM:Several seasonal variations have been found in birth rates in different countries at different periods. The characteristics of the rhythmic patterns vary according to geographical location and chronological changes. This study presents data on spanish birth seasonality over six decades. METHODS:A time series composed of 33,421,731 births in Spain in the period 1941-2000 was analysed. The series comes from the National Institute of Statistics and was processed according to the following norms: (1) normalisation of the duration of months and years; (2) clinical analysis of temporal series (isolation of seasonal component); (3) Fourier's spectral analysis; and (4) cosinor analysis (adjustment to the cosine curve of two harmonics). RESULTS:Significant seasonal rhythm was found in the set of births, both for a 12-month period and a 6-month period. The rhythm shows bimodal morphology, with a pronounced birth peak in April and a smaller one in September. These peaks correspond to July and December conceptions, respectively. The major birth peak shifted to March-May between the 1940s and the 1980s. Birth rhythm changed after the 1960s, with a decrease in amplitude and later loss of seasonality in the 1990s. CONCLUSIONS:In Spain, seasonal birth rhythm shows a decline from 1970, and, finally, lack of birth seasonality in 1991-2000. This trend is similar to other European countries, although Spain shows a more intense loss of seasonality.