Background The impact of very early castration of foals has not yet been studied despite the many positive effects observed in dogs and cats. Objectives The objective of the study was to compare castration at 3 days and 18 months and assess their subsequent morphological and behavioural development. Study design This was a randomised, blinded clinical study. Methods Twenty-two Welsh ponies underwent either early (3-day old, EC group, n = 11) or traditional (18-month old, TC group, n = 11) castration. Animals were followed up to 3 years of age. All ponies were castrated using a primary closure technique under general anaesthesia. Weight and morphometric measurements were monitored monthly from birth until 8 months of age in both groups. Then, measurements were taken every 3 months until 2 years of age and then every 6 months until 3 years of age. Temperament tests were performed on all animals when they were 1- and 3-years old. Results No differences were observed between the EC and TC groups in terms of physical development from birth until 40 months of age or in terms of temperament and behaviour at either 1 or 3 years of age. Main limitations The study included only one breed (Welsh ponies) and only 22 animals that were castrated before 2 years of age, precluding comparison with castration performed at older ages. Conclusions We demonstrate that early castration at 3 days does not interfere with morphological or behavioural development.