ObjectiveTo investigate the heterogeneity of gut microbiota between patients with solitary pulmonary nodules (SPN) and multiple pulmonary nodules (MPN), and to explore the intrinsic relationship between Traditional Chinese Medicine (TCM) constitution types and the intestinal microecology. MethodsA prospective study was conducted on 280 patients with pulmonary nodules enrolled between April 2022 and December 2024 from Hospital of Chengdu University of Traditional Chinese Medicine, Sichuan Cancer Hospital, Chengdu Integrated Traditional Chinese Medicine & Western Medicine Hospital. Among them, 118 (42.1%) were male and 162 (57.9%) were female, with a median age of 50 (42, 57) years. Based on imaging findings, patients were divided into a SPN group (n=65) and an MPN group (n=215). TCM constitution types were identified using a Constitution in Chinese Medicine Questionnaire. Fecal samples were collected for 16S rRNA sequencing. Bioinformatics analysis was employed to analyze inter-group differences in microbial community structure. The correlation between TCM constitutions and gut microbiota was examined using Procrustes analysis and Spearman correlation analysis. ResultsThe distribution of TCM constitution types between the two groups showed a statistically significant difference (P<0.05). The SPN group was predominantly characterized by the Qi-depression constitution, while the MPN group was more commonly associated with Yang-deficiency and Phlegm-dampness constitutions. Microbiota analysis revealed that the gut microbiota health index was significantly higher in the SPN group than in the MPN group (P<0.05), whereas the microbiota dysbiosis index showed the opposite pattern. Taxonomic analysis identified higher abundances of Ruminococcus_torques_group, Haemophilus, and Fusobacterium in the SPN group. The abundance of Leuconostoc was significantly increased in the MPN group. Procrustes analysis and Spearman correlation analysis indicated that in the SPN group, the Qi-depression constitution was positively correlated with Ruminococcus_torques_group and Bacteroides. In the MPN group, the Yang-deficiency constitution was negatively correlated with Faecalibacterium, while no statistically significant correlations were found between specific bacterial genera and the Phlegm-dampness or Qi-deficiency constitutions. ConclusionSPN and MPN exhibit significant heterogeneity in TCM constitutional tendencies and microecological characteristics. The abundance of specific bacterial genera may serve as potential biomarkers for distinguishing between SPN and MPN. The interaction between TCM constitutions and specific gut microbiota provides a theoretical basis for the precise TCM syndrome differentiation and microecological intervention in pulmonary nodules.