The effect of high concentrations of airborne dust on the health and performance of growing pigs (55.82 kg) was determined by introducing mechanically ground fecal dust into the air space of the pigs in the first trial and feed dust in the second trial. Mean respirable dust levels during the light hours in the rooms injected with feed and fecal dust were 111 and 56 particles/ml, respectively, while the mean level in the control rooms was 25 and 27 particles/ml, respectively. Lungs from 115 animals were collected during slaughter. No relationship was found between dust concentration and lung score or between dust concentration and average daily gain. For the fecal and the feed dust trials, 38% and 23% of the pigs had pneumonic involvement in over 10% of the lung. The average daily gain was not significantly different between treatments (mean 0.78 kg/d) and it was not affected significantly by the percentage of pneumonic involvement. A large amount of energy was required to reduce feed to the respirable range relative to that required for fecal dust suggesting that most of the respirable dust is of fecal origin.