A recent retrospective cohort study of 1006 subjects reported a marked reduction in condom failure amongst users applying additional lubrication. Improving the reliability of condoms would reduce the incidence of unwanted pregnancies and STI's (and hence health and social costs). This trial investigates the relative risk reduction amongst couples applying additional spermicidal lubrication to condoms, other factors associated with condom failure, the acceptability and side effects of additional spermicide and its cost effectiveness.