Trimethylamine oxide (TMAO), a metabolite of intestinal microorganisms, has been much focused on a marker of ischemic injury in recent years. With deepening research on the intestinal flora, researchers have found that TMAO is closely related to chronic diseases, especially cardiovascular and cerebrovascular diseases, diabetes, and chronic kidney disease. These diseases usually feature a long latent period, no obvious clinical symptoms, irreversible course, and poor prognosis. Therefore, early detection and treatment of these chronic diseases are particularly important. This article briefly summarizes the relationship between TMAO and the above chronic diseases and the main methods used to determine TMAO with a view of providing theoretical basis for early intervention of such chronic diseases.