Recurrent Clostridium difficile–associated diarrhea (RCD) is associated with reduced diversity of the fecal microbiome. This study compared the fecal microbiota of patients with RCD, initial CDAD (ICD), and healthy controls using 16S rRNA gene sequencing. Patients with RCD showed significantly lower microbial diversity and richness compared to ICD and control subjects. The microbial communities in RCD patients were more variable and deviated from the normal dominance of Bacteroidetes and Firmicutes. In contrast, ICD patients and controls had similar microbial diversity. The reduced diversity in RCD patients may contribute to the failure of colonization resistance against C. difficile, leading to recurrent infections. The study highlights the importance of maintaining microbial diversity in preventing and treating RCD. Current treatments for RCD, such as antibiotic therapy or fecal microbiota transplantation, aim to restore microbial balance. However, the role of microbial diversity in RCD remains understudied. This research provides molecular evidence that decreased microbial diversity is a key factor in RCD, suggesting that strategies to restore microbial diversity could be effective in preventing and treating this condition. The findings emphasize the need for further research into the microbiome's role in RCD and the development of new therapeutic approaches.Recurrent Clostridium difficile–associated diarrhea (RCD) is associated with reduced diversity of the fecal microbiome. This study compared the fecal microbiota of patients with RCD, initial CDAD (ICD), and healthy controls using 16S rRNA gene sequencing. Patients with RCD showed significantly lower microbial diversity and richness compared to ICD and control subjects. The microbial communities in RCD patients were more variable and deviated from the normal dominance of Bacteroidetes and Firmicutes. In contrast, ICD patients and controls had similar microbial diversity. The reduced diversity in RCD patients may contribute to the failure of colonization resistance against C. difficile, leading to recurrent infections. The study highlights the importance of maintaining microbial diversity in preventing and treating RCD. Current treatments for RCD, such as antibiotic therapy or fecal microbiota transplantation, aim to restore microbial balance. However, the role of microbial diversity in RCD remains understudied. This research provides molecular evidence that decreased microbial diversity is a key factor in RCD, suggesting that strategies to restore microbial diversity could be effective in preventing and treating this condition. The findings emphasize the need for further research into the microbiome's role in RCD and the development of new therapeutic approaches.