Outcome indicators will include knowledge about HIV and AIDS, sexual behaviour indicators - partner acquisition and partnership duration, mixing patterns and condom use - incidence of HIV and selected STD infections, and health-seeking behaviour indicators. Sample sizes were calculated to give an 80% power at the 95% significance level, comparing the matched intervention and comparison clusters, for a range of different outcome variables. The formulae used in these calculations take account of possible between-cluster and - in the case of prevalence - within-cluster variability [22,28]. Given the level of spatial mobility which exists within the Honde Valley area, some diffusion effect is likely to be present. This has been reflected in the sample size computations through the use of conservative anticipated excess reductions in the intervention clusters. We estimate that a sample size of 1,000 individuals per cluster will yield a minimum of 700 observations, after accounting for loss to follow-up. In the case of the HIV incidence calculations, these observations will provide 1,120 susceptible person-years of observation, after allowing for a baseline prevalence in the general adult population of 20%. Under these conditions, an initial sample size of 1,000 per cluster should be sufficient to permit detection of a 40% excess reduction in HIV incidence, given a baseline annual incidence of 2% or higher. This sample size should also enable us to detect, inter alia, a 40% lower incidence of new HSV2 and trichomoniasis cases among individuals not infected at baseline and a 40% reduction in reported casual relationships without the use of condoms, given a baseline prevalence of 10% or over. Evidence from model simulations and empirical studies suggests that the excess reduction in HIV incidence required is plausible given a combined intervention strategy [19,29]. Furthermore, the required excess reductions in STDs and high-risk behaviour should be attainable. Reductions in these factors, of the order indicated, would, if demonstrated, represent strong indirect evidence that the intervention strategy is slowing the spread of HIV infections. Reduction in other STDs would be a significant benefit, in itself, given the morbidity and adverse fertility consequences they frequently cause.