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, 35 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.