Summary. The resurgence of tuberculosis (TB) in New York City in the period 1978-92 has been closely linked to the AIDS epidemic but the increase of active TB in areas of urban poverty also implies increased community exposure. We have examined the ecological relation between community rates of AIDS and residential crowding and cases of active TB in Bronx children under age 5. Residential crowding was defined as the percent of households with more than 1 person per room. All childhood TB cases reported between 1986 and 1992 for the Bronx (n=75) were included. Cumulative AIDS mortality rates for adult females through 1990 represented community HIV burden. All data were coded by the 64 health areas of the borough. We examined trends in these data and used Poisson regression to model the effect of HIV burden and residential crowding on TB risk. For the Bronx as a whole the two variables of TB and residential crowding showed a clear temporal correspondence for the period 1970-90. Residential crowding was associated with poverty and greater dependence on public assistance, large household size, Hispanic ethnicity, and a higher proportion of young children. The overall TB case rate increased with the proportion of crowded households, with a rise from 1 47 to over 8 cases per 10 000 children as the proportion of crowded households increased. At both the lowest and highest levels of AIDS mortality in these areas, the childhood TB risk increased as crowding increased. Children living in areas of the Bronx in which over 12 percent of homes are severely overcrowded were 5·6-fold more likely to develop active TB, even after holding constant the presumed HIV burden in each local community. While HIV infection, the newest risk factor for TB, appears to play a critical role in the resurgent epidemic, our findings show that the earliest known TB risk factors, poverty and household crowding, are still potent forces.