To determine whether bacteria present in ambient air play a role in the contamination of fine-particle reservoir nebulizers, nebulizers were placed in operation in separate hospital locations having qualitatively and quantitatively different bacterial flora in background air. Nebulizers placed in a surgical intensive care unit that had higher numbers of bacteria and a predominance of Gram-negative organisms in background air had a significantly higher incidence of nebulizer contamination (33.0%) than did nebulizers placed in a non-patient-care area that had lower bacterial counts and a predominance of Gram-positive organisms (0%) (P less than .05). The present study indicates that airborne contamination of fine-particle reservoir nebulizers occurs when bacteria present in ambient air enter the nebulizer during its operation.