In an athymic rat model of chronic Pseudomonas aeruginosa lung infection mimicking cystic fibrosis (CF), we studied the effects of the Chinese herb ginseng. Rats were treated subcutaneously with ginseng extracts (25 mg/kg) once a day for 10 days after challenge with P. aeruginosa embedded in alginate beads. We found that ginseng treatment significantly reduced bacterial load (p < 0.02) and the number of mast cells in the lungs (p < 0.01). Furthermore, it decreased the severity of lung pathology (p < 0.02) and lowered serum anti-P. aeruginosa IgM and IgA antibody levels (p < 0.004, p < 0.04) compared to the control group. The down-regulated specific humoral immunity in the ginseng-treated group and the fact that athymic rats have a severely compromised T-cell-mediated immune reactivity due to the absence of thymus might suggest an activation of innate immunity after ginseng treatment. Our findings indicate that ginseng treatment increases the resistance of the athymic rats to P. aeruginosa lung infection. We therefore think that ginseng has promising potential as a natural medicine for stimulation of the immune system in CF patients with chronic P. aeruginosa lung infections.