Partner studies of heterosexual transmission of HIV have observed tranmissions after relatively few sexual contacts and couples who have remained discordant, with respect to HIV, whilst considered to be at high risk over prolonged periods, suggesting huge variation between individuals in whether a contact seroconverts. This paper is based on a longitudinal partner study which aims to identify behavioural and biological factors which influence heterosexual transmission of HIV. The index case (the first infected) is defined as a patient who is HIV positive whilst the contact partner is a person of the opposite sex who has had a sexual relationship with the index case. In Edinburgh, from October 1987 to the beginning of June 1992, one-hundred and twenty couples have been recruited where the contact's risk of infection is only through heterosexual intercourse with his/her index case. At recruitment, 24 couples (20%) were concordant with respect to HIV and since recruitment one contact has seroconverted. At the initial interview the contact is asked about her/his past sexual practices and contraceptive use and counselled about safer sex. Follow-up interviews of negative contacts take place to reassess these behavioural data and their HIV status. Biological data on the index is also available as the majority of cases are in clinical care.

Three factors which might influence heterosexual transmission of HIV are to be assessed:

• behavioural aspects of the couple,

• infectivity of the index,

• susceptibility of contact.

Behavioural data are required to confirm that the virus has an ‘opportunity’ to transmit. Virological and immunological factors form the basis for assessing the infectivity of the index and susceptibility of the contact.