We assessed the relationships between annual height velocity, calculated from two measurements taken at intervals of 6, 12 and 24 months, in 69 healthy prepubertal children (34 male, 3 5 female) aged 6.5 years (range 6.25-6.75 yrs) who were followed for 2.5 years and measured biannually. Initial height values for age were within normal ranges except for one child with a height SDS of - 2.0. Mean annual velocities also conformed to British norms. Significant differences were observed between consecutive six-monthly velocities and these values failed to predict annual velocities calculated from measurements taken at 12- or 24-month intervals. Seasonal effects were thought to have some impact on the six-monthly velocity differences. Height velocity, based on measurements taken at intervals of 12 months, was essentially similar to two-year velocity values. The results indicate that in estimating annual height velocity, six-month intervals between measurements may be misleading while 12-monthly measurements are as reliable as height velocity values based on measurements taken at two-year intervals.