Cardiovascular diseases (CVDs) including heart failure (HF) is a major health, medical and social issue that needs to be resolved in Japan's super-aged society. Recent clinical and basic studies suggest that the gut microbiota and their metabolites play critical roles in the onset and progression of CVDs. We explored changes in gut microbiota composition and metabolite levels among Japanese patients to investigate their association with CVDs. Changes in specific bacteria were observed, with a decrease in phylum Bacteroidetes and increases in order Lactobacillus or genus Streptococcus in coronary artery disease patients. For HF patients, a reduction in phylum Bacteroidetes and increases in phylum Actinobacteria (e.g. Bifidobacterium) and Proteobacteria (e.g. Escherichia, Shigella, and Klebsiella) were noted. Elevated levels of gut microbiota-associated metabolites, such as trimethylamine N-oxide (TMAO) and indoxyl sulfate, were observed in CVD patients, suggesting potential effects on organ functions. Many studies have linked higher plasma TMAO levels to worse prognoses in CVDs, including HF and renal failure. However, the clinical significance and therapeutic potential of these findings require further investigation. In this manuscript, the author aims to review the current status of research on gut microbiota in CVDs, with a primary focus on the microbes themselves and their related metabolites. Further research is essential to comprehensively understand these intricacies and establish clear cause-and-effect relationships, ultimately paving the way for the development of innovative therapies for CVDs.