The aim of the study was evaluation of the correlation between the level of clinical activity of Crohn's disease (CD) and the number of Faecalibacterium prausnitzii, total number of bacteria and the concentration of selected short chain fatty acids (SCFA) in stool. 34 individuals diagnosed with Crohn's disease participated in this study in 2011. The disease activity was determined according to the Crohn Disease Activity Index (CDAI). The number of Faecalibacterium prausnitzii and total number of bacteria were monitored by RT-PCR. The concentrations of SCFA were determined by gas chromatography. In CD patients, Faecalibacterium prausnitzii number and percentage of the total number of bacteria were greatly reduced. In patients with CD the percentage of acetate was elevated (70%), while the percentages of propionate and butyrate were significantly reduced (14.9% and 7.99%, respectively).