The balance between beneficial and pathogenic bacteria in the intestine has a great importance in terms of gut physiology and immunity. The aim of the study was to investigate the stimulatory and inhibitory effects of aldehydes from the green leaf volatiles family (trans-2-hexenal, cis-3-hexenal, trans-2-nonenal, and trans-2-decenal) on beneficial (Bifidobacterium bifidum, Bifidobacterium longum subsp. infantis, Lactobacillus acidophilus, and Lactobacillus casei) and pathogenic bacteria (Fusobacterium nucleatum subsp. nucleatum, Clostridium perfringens, Staphylococcus aureus subsp. aureus, Escherichia coli, and Salmonella enterica subsp. enterica serovar Typhimurium) from the intestine. The growth of B. bifidum was stimulated by trans-2-hexenal and trans-2-decenal at 3.9-250 mu g/mL, by cis-3-hexenal at 15.6 and 31.3 mu g/mL, and by trans-2-nonenal at a dose of 3.9-500 mu g/mL (p < 0.05). Trans-2-decenal also moderately stimulated L. acidophilus at concentrations of 31.3 and 62.5 mu g/mL (p < 0.05). Trans-2-hexenal, cis-3-hexenal, and trans-2-nonenal did not inhibit beneficial intestinal bacteria, with the exception of B. infantis. Trans-2-decenal was the most effective aldehyde on pathogens, with growth-inhibitory effect on C. perfringens, F. nucleatum, and S. aureus at the concentration of 500 mu g/mL. Trans-2-decenal also protected beneficial bacteria at the dose at which it inhibited pathogenic ones. All the used aldehydes at a concentration of 500 mu g/mL inhibited the growth of F. nucleatum as one of the agents of colorectal cancer. Among the pathogens, E. coli and S. Typhimurium were resistant to all aldehydes while S. aureus was inhibited only by trans-2-decenal. In conclusion, the use of aldehydes from the green leaf volatiles family might have beneficial effects on gut health by regulating beneficial bacteria and inhibiting pathogenic bacteria.