Attempts were made to produce twin calves by implanting in vitro fertilised and matured (IVF) embryos into dairy cows. Twinning rates were consistently at the lower end of the range reported to have been obtained with in vivo derived embryos. The low pregnancy rates, higher than expected early embryonic losses, and increased fetal deaths and abortions recorded may indicate that IVF embryos are less viable. Fetal oversize in single calves was a significant problem in the first two years of the trial. The subsequent conception rates of cows that calved twins and those that calved singles were similar in spite of a much higher incidence of retained placenta and an increased interval to first oestrus in the cows bearing twins.