INTRODUCTION: Depressive disorder has been conceptualised as a condition of accelerated biological ageing. We operationalized a frailty index (FI) as marker for biological ageing aimed to explore the bidirectional, longitudinal association between frailty and either depressive symptoms or depressive disorder. METHODS: - A cohort study with 6-year follow-up including 377 older (≥60 years) outpatients with a DSM-IV defined depressive disorder and 132 never-depressed controls. Site visits at baseline, two and six-year follow-up were conducted and included the CIDI 2.0 to assess depressive disorder and relevant covariates. Depressive symptom severity and mortality were assessed every six months by mail and telephone. An 41-item FI was operationalized and validated against the 6-year morality rate by Cox-regression (HRFI =1.04 [95% CI: 1.02 - 1.06]). RESULTS: Cox-regression showed that a higher FI was associated with a lower chance of remission among depressed patients (HRFI =0.98 [95% CI: 0.97 - 0.99]). Nonetheless, this latter effect disappeared after adjustment for baseline depressive symptom severity. Linear mixed models showed that the FI increased over time in the whole sample (B(SE)=0.94 (0.12), p<.001) with a differential impact of depressive symptom severity and depressive disorder. Higher baseline depressive symptom severity was associated with an attenuated and depressive disorder with an accelerated increase of the FI over time. CONCLUSIONS: The sum score of depression rating scales are likely confounded by frailty. Depressive disorder according to DSM-IV criteria is associated with accelerated biological ageing. This argues for the development of multidisciplinary geriatric care models incorporating frailty to improve the overall outcome of late-life depression.