Defects in polymorphonuclear neutrophil (PMN) adherence and chemotaxis in neonates are thought to be an important cause of their increased susceptibility to overwhelming bacterial infection. Few studies of these functions have been carried out in stressed neonates who are at even greater risk of infection. PMN adherence and chemotaxis were examined in 33 stressed neonates with acute lower respiratory illness, 13 healthy neonates, and 43 healthy adults using whole blood PMN adherence and chemotaxis assays. PMN chemotaxis was significantly decreased in stressed neonates (locomotion index of 38.4 ± 9.7 µm) compared with that of healthy neonates (48.9 ± 12.8 µm,p<0.01) or adults (61.6 ± 11.9 µm,p<0.001). PMN chemotaxis was studied during illness and recovery in 13 of the 33 stressed neonates and showed significant improvement during recovery (41.6 ± 9.9 and 53.2 ± 11.9 µm, respectively,p= 0.012). PMN adherence was decreased in stressed neonates (1.4 ± 1.6%) compared with that of adults (12.3 ± 11.4%,p<0.01) but was similar to that of healthy neonates (1.1 ± 1.4%). These findings suggest that further impairment of PMN chemotaxis in stressed neonates helps account for their increased susceptibility to overwhelming bacterial infection.