Objective To investigate whether concomitant Alzheimer's disease (AD) pathology, reflected by cerebrospinal fluid (CSF) biomarkers, has an impact on dementia with Lewy bodies (DLB) in terms of clinical presentation, cognitive decline, nursing home admittance and survival. Participants We selected 111 patients with probable DLB and CSF available from the Amsterdam Dementia Cohort. On the basis of the AD biomarker profile (CSF tau/amyloid-β 1-42 (Aβ42) ratio >0.52), we divided patients into a DLB/AD+ and DLB/AD- group. Of the 111 patients, 42 (38%) had an AD CSF biomarker profile. We investigated differences between groups in memory, attention, executive functions, language and visuospatial functions. Difference in global cognitive decline (repeated Mini-Mental State Examination (MMSE)) was investigated using linear mixed models. Cox proportional hazard analyses were used to investigate the effects of the AD biomarker profile on time to nursing home admittance and time to death. Results Memory performance was worse in DLB/AD+ patients compared with DLB/AD- patients (p<0.01), also after correction for age and sex. Hallucinations were more frequent in DLB/AD+ (OR=3.34, 95% CI 1.22-9.18). There was no significant difference in the rate of cognitive decline. DLB/AD+ patients had a higher mortality risk (HR=3.13, 95% CI 1.57 to 6.24) and nursing home admittance risk (HR=11.70, 95% CI 3.74 to 36.55) compared with DLB/AD- patients. Conclusions DLB-patients with a CSF AD profile have a more severe manifestation of the disease and a higher risk of institutionalisation and mortality. In clinical practice, CSF biomarkers may aid in predicting prognosis in DLB. In addition, DLB-patients with positive AD biomarkers could benefit from future treatment targeting AD pathology.