PURPOSE: Loneliness is a prevalent affective issue among patients with breast cancer, with its developmental trajectory being a contentious subject. Therefore, the aim of this study was to explore trends in loneliness in patients with breast cancer and identify predictors of different trajectory categories. METHODS: Using convenience sampling, 176 patients planning to undergo breast cancer surgery in a university hospital in Shaanxi Province, China, were followed up six times over 12 months following surgery, and data from 144 patients were analyzed. The data were analyzed using a mixed growth model (GMM) and logistic regression. RESULTS: Two latent classes of loneliness trajectory were identified among patients with breast cancer, namely "persistent high loneliness" and "persistent low loneliness." Patients who with education level of junior secondary and less (OR = 13.59, P = 0.002), had a melancholic temperament (OR = 12.07, P = 0.002) were more likely to be categorized in the "persistent high loneliness group", whereas the better family functioning (OR = 0.60, P <
0.001) and choleric temperament (OR = 0.16, P = 0.025) of the patients were more likely to be categorized in the "persistent low loneliness group". CONCLUSION: Patients with breast cancer exhibit diverse trajectories of loneliness, with educational level, temperament type, and family functioning being predictive of these trajectories. Therefore, it is crucial to promptly identify populations at risk in a clinical setting and devise intervention strategies, grounded in identified trajectory characteristics and influencing factors, to enhance patient outcomes.