The Brazilian lightning detection network detected about 310,000 cloud-to-ground flashes over three large metropolitan areas in Southeastern Brazil during three summer seasons (from 2000 to 2002). It was observed an enhancement of 60--100% in the flash density over the urban areas compared to their surroundings. Conversely, a decrease of 7--8% in the percentage of positive flashes was also observed. The spatial distribution of the flashes follows closely the shape of the S¿o Paulo city heat island, thereby supporting the thermal hypothesis. The PM10 concentration showed a positive linear correlation with the number of cloud-to-ground flashes and a negative correlation with the percentage of positive flashes, supporting the aerosol hypothesis. These results indicate that both hypotheses should be considered to explain the CG lightning enhancements over the urban areas. However, only the aerosol hypothesis could better support the decrease of the percentage of positive flashes.