The Italian standards of birth weight have been computed using information routinely collected by the Italian Central Institute of Statistics on about 1,200,000 births in the period 1984-1985. Individual records include data on birth weight and main fetal and maternal characteristics and delivery modalities. The crude and smoothed 5th, 10th, 50th, 90th and 95th centiles of weight at birth as function of gestational age according to sex, type of birth, maternal age and parity are presented. Centiles of weight at birth were higher (about 5%) in males than in females in all gestational ages: for example the 50th centile of weight at 40 weeks gestation was 3479 g in males and 3332 g in females. Between the 28th and the 32nd week the 50th centile of birth weight for multiple births grew at a rate similar to that of singletons; but beyond 32 weeks the weight growth in multiple birth was markedly lower than in singleton ones, the median multiple birth weight reaching the 10th centile of singleton at 38 weeks. The values of centiles increased with parity in both sexes and all gestational ages. The difference was however limited: for example with reference to the 50th centile the value for births in women reporting three or more births was about 5% higher than in those reporting no previous birth. Likewise, centiles of weight were higher in older women, but the difference tended to disappear after 36 weeks gestation. This analysis shows from a large national data-set standards of weight at birth from a Southern European population, providing to obstetrics and perinatologists curves of fetal growth more directly representative of the population under their care.