Rearing heifer replacements that reach first calving at an age and body weight (BW) to achieve their full lifetime potential, in terms of both yield and profitability, is a key factor in dairy enterprises. Rate of growth during the rearing period has a direct effect on age at first calving (AFC) (Heinrichs, 1993). It is widely accepted that the average daily gain (ADG) of Holstein-Friesian heifers should be approximately 0.7 kg/d before puberty, and 0.8 kg/d after. However, we have previously found extreme variability in the ADG of calves both within and between commercial dairy farms; this may in part be due to the very limited use of weight or height measurements on most farms. The aim of the present study was to determine the effect of growth rate during the first six months of life on AFC, and subsequent fertility and milk production during first lactation of Holstein-Friesian heifers on UK dairy farms.