We use a microsimulation model to estimate the proportions of rural Malawian brides and grooms who are already HIV positive when they marry. The model, a demographic model of reproduction and mortality overlaid with a model of disease transmission, incorporates behavioural input data derived from the second round of the Malawi Diffusion and Ideational Change Project, which was conducted in three areas of rural Malawi in 2001. We estimate that HIV infection is present in between 13 and 20 per cent of couples. Although young women are more likely to be HIV positive than men of the same age, as a result of their low ages at marriage only around two per cent of brides are estimated to be HIV positive.