OBJECTIVE: Cancer-related fatigue (CRF) is among the most distressing symptoms reported by cancer survivors as compromising their quality of life. This study investigates the complex interplay between CRF and functional health (FH) in survivors of Hodgkin lymphoma by using longitudinal data to clarify the etiology of CRF. METHODS: Data from N = 3596 survivors (HD13-15) from the German Hodgkin Study Group was analyzed using bivariate latent curve models with structured residuals to model how the interplay between CRF and FH unfolds over time across and within individuals. CRF and FH were measured with the EORTC QLQ-C30. Assessed FH domains were physical, cognitive, emotional, social, and role functioning. Age at diagnosis, gender, country, baseline fatigue, and cancer stage were included as covariates. RESULTS: The latent curve models with structured residuals had an adequate model fit (χ² = 416.63-548.28, df = 114, p < .001, root mean square error of approximation = .03, comparative fit index = .98-.99, Tucker-Lewis index = .97-.98). On the between-person level, CRF and all FH domains were strongly negatively correlated (r = -.72 to r = -.84). On the within-person level, earlier CRF (ρF = -.05 to ρF = -.12) and FH deviations (ρFH = -.05 to ρFH = -.13) negatively predicted subsequent deviations of the respective other variable. Time-specific within-person improvements in physical, cognitive, and emotional functioning reduced CRF more than vice versa, whereas the effect of CRF was stronger for social functioning. Role functioning had a balanced relation with CRF. CONCLUSIONS: This analysis reveals a complex reciprocal relation between CRF and FH with distinct between- and within-person effects. The results contribute to a better understanding of CRF in survivors of Hodgkin lymphoma and could inform the development of much-needed targeted interventions. (PsycInfo Database Record (c) 2020 APA, all rights reserved).