Stigmatising attitudes towards people living with HIV and AIDS (PLHIV) are hampering attempts to control HIV epidemics in sub-Saharan African countries. This study measures the effect of social capital, in the form of local community groups, in reducing stigma and tests a new explanatory framework for the association between community group membership and less stigmatising attitudes. Prospective data on membership of a wide range of different community groups and stigmatising attitudes (being unwilling to care for a relative with AIDS), collected from a general population cohort of 5,253 men and women aged 15-54 years in eastern Zimbabwe between 2003 and 2008 were analysed using multivariable logistic regression. 36 % of respondents were members of community groups throughout the study period. Individuals in community groups were less likely to express stigmatising attitudes towards PLHIV-3.4 versus 9.5 % (adjusted odds ratio = 0.46, p < 0.001). Discussions of care for PLHIV within groups, improved knowledge about AIDS, greater exposure to PLHIV, and increased uptake of HIV testing and counselling did not account for the association. Further work is needed to identify the mechanisms through which community participation can reduce stigma. Nevertheless, these findings suggest that promoting well-informed discussions about HIV within pre-existing community groups and involving these groups in stigma reduction programmes could be effective means of reducing stigma at the grassroots level.