Depression is common in chronic illness, albeit prevalence can be highly variable. This variability may be a function of symptom overlap between depression and chronic illness. Using Obstructive Sleep Apnoea (OSA) as an exemplar, this meta-analysis explored whether the proportion of overlapping symptoms between OSA and depression, within different depression questionnaires, moderates prevalence estimates.A systematic search identified 13 studies meeting eligibility criteria.Based on depression questionnaires, the prevalence of depression in OSA ranged from 8% to 68%, reflecting marked heterogeneity. Prevalence estimates based on questionnaires with greater symptom overlap between OSA and depression were higher, whereas questionnaires with a higher proportion of anhedonia symptoms were associated with lower prevalence estimates.Overall, these data suggest that when using depression questionnaires to assess the prevalence of depression in OSA, questionnaires that have a lower proportion of symptom overlap between OSA and depression, as well as a higher proportion of anhedonia symptoms, reduce the likelihood of overestimating the prevalence of depression in OSA. This study has implications for other chronic illnesses with symptom overlap with depression, for example diabetes, chronic kidney disease, or heart disease, as well as suggesting that depression questionnaires are not equally appropriate for assessing depression symptomatology in chronic illness populations.