Continuous measurements of the size distribution of atmospheric aerosol in the 3--48 nm diameter range were performed in the Mexico City metropolitan area. These measurements were made during the period 10--20 April 2003 at a ground-based, mountain pass site in the southeast corner of the Mexico City Federal District and during the period 2--11 May 2003 at the CENICA site located near the city center. The objectives of this work were to determine the frequency of new particle formation and to characterize the atmospheric chemical and meteorological conditions that lead to these events. Several new particle formation events were recorded during the study. Events observed in the mountain pass correlate with northerly winds and elevated levels of sulfur dioxide in the mid-morning while events observed in the city correlate with elevated concentrations of sulfur dioxide and low particulate matter mass concentrations in the afternoon hours.