Mycobacterium avium subsp. paratuberculosis(Mptb) causes Johne's disease in ruminants. Disease control programs aim to break the faecal-oral cow-calf transmission cycle through hygienic calf rearing and removal of cows shedding Mptb from the herd, but these programs do not take account of congenital infection. The aims of this study were to determine the prevalence of foetal infection in cattle and to estimate the incidence of calves infected via the in utero route following meta-analysis. 9% (95% c.l. 6-14%) of foetuses from subclinically infected cows and 39% (20-60%) from clinically affected cows were infected with Mptb. The true rates of infection would be higher than these figures suggest due to incomplete sensitivity of culture methods. The incidence of calf infection derived via the in utero route was estimated to be in the range 0.44 to 1.2 infected calves per 100 cows p.a. in herds with within-herd prevalence of 5% and 3.5-9.3 in herds with 40% prevalence. In utero transmission of Mptb could retard the success of disease control programs if the opportunities for post natal transmission via colostrum/milk and environmental contamination were able to be controlled. The immunological consequences of foetal infection will be discussed in the context of diagnosis and vaccination.