The effect of twinning on the subsequent health, production and reproductive performance of dairy cattle was studied by analysing the data derived from 19,755 calvings which occurred during three years on 37 farms. The data formed part of the database of a veterinary practice operating the DAISY dairy cow recording scheme for its dairy farmer clients. The average twinning rate was 2.5 per cent. For first calf heifers the rate was 0.9 per cent, and the rate increased with increasing parity to over 5 per cent for cows calving for their sixth and subsequent lactations. Although they produced more milk than their contemporaries, twin-bearing cows suffered an increased incidence of retained placenta and vulval discharges and their calving to conception interval was extended by 33 days. Furthermore, 35 per cent of these cows were culled compared with 21 per cent of their contemporaries. The benefit of having more calves for sale was reduced owing to 15 per cent of them being born dead. It is calculated that producing twins resulted in an average loss of income of 74 pounds/cow, a deficit of 15 per cent compared with cows having single calves.