Rationale: Chronic exposure to air pollution has been associated with adverse effects on children's lung growth. Objectives: We analyzed the effects of chronic exposure to urban levels of particulate matter (PM) on selected phases of mouse lung development. Methods: The exposure occurred in two open-top chambers (filtered and non-filtered) placed 20m from a street with heavy traffic in Sao Paulo, 24 hours/day for eight months. There was a significant reduction of the levels of PM_2.5 inside the filtered chamber (filtered=2.9±3.0 µg/m³, non-filtered=16.8±8.3 µg/m³; p=0.001). At this exposure site, vehicular sources are the major components of PM_2.5 (PM2.5Km). Exposure of the parental generation in the two chambers occurred from the 10th to the 120th days of life. After mating and birth of offspring, a cross-over of mothers and pups occurred within the chambers, resulting in four groups of pups: non-exposed, prenatal, postnatal and pre+postnatal. Offspring were euthanized at the age of 15 (n=42) and 90 (n=35) days; lungs were analyzed by morphometry for surface to volume ratio (as an estimator of alveolization). Pressure-volume curves were performed in the older groups, using a 20ml plethysmograph. Results: Mice exposed to PM_2.5 pre+postnatally presented a smaller surface to volume ratio when compared to non-exposed animals (p=0.036). The pre+postnatal group presented reduced inspiratory and expiratory volumes at higher levels of transpulmonary pressure (p=0.001). There were no differences among prenatal and postnatal exposure and non-exposed animals. Conclusions: Our data provide anatomical and functional support to the concept that chronic exposure to urban PM affects lung growth.