The records of 453260 British Friesian cows, accumulated by the Milk Marketing Board of England and Wales in 1981/82, were used to estimate the effect of the calving interval on milk production in the subsequent lactation. There was a clear relationship between the length of the dry period and the calving interval in which it occurred, such that the dry period was extended by 2·5 d for every 10 d extension of the calving interval. The relationship between the interval and milk production in the following lactation appeared more likely to be due to variation in the dry period than to variation in the calving interval itself. A dry period between 50 and 60 d was associated with maximum production of milk, fat and protein. Among cows with dry periods shorter than 55 d, fat and protein production increased by 8 and 3·5 kg/d respectively. For cows with dry periods longer than 55 d, production of fat and protein was reduced by 0·2 and 0·12 kg/d respectively. The modal calving interval was 355 d including a dry period of 51 d. The interval tended to increase with the age of the cow, at the rate of 3 d/month of age within parity. The best estimate of future calving intervals was the mean of all previous intervals of the cow.