This study analyses gestational age, mode of delivery and size at birth in children who later developed idiopathic or organic growth hormone deficiency (GHD). A data register of children on growth hormone (GH) treatment in Sweden was compared with the Swedish Medical Birth Register during a 14-year period (1973-1986) comprising 1.4 million newborn children. Size at birth was evaluated using a new Swedish reference standard based on data from around 500,000 newborn children. It was found that the children who later develop idiopathic GHD (IGHD) were born with a normal distribution of gestational age. They were more often born with breech delivery (7.1% versus 2.8%) or caesarean section (16.6% versus 10.4%) compared with normal children. The children's condition at birth was poorer than normal, as shown by the frequency of Apgar scores below 7 at 5 minutes (5.2% versus 1.2%). Finally, it was found that children who later develop IGHD (n = 220) had a median birth length of 0.87 SDS below the mean and a median birth weight of 0.60 SDS below the mean of the standard. In contrast, both the birth length and weight of the children who later develop organic GHD (OGHD) (n = 92) did not differ from that of the reference.