OBJECTIVE:To determine the influence of comorbidity on physical function in osteoarthritis (OA) consulters aged 50 years and over in family practice. METHODS:The study design linked morbidity consultations during an 18-month period to self-reported physical function status measured at the end of the period. Clinical comorbidity was compared between consulters with (n = 1026) and without (n = 8160) OA. Comorbidity was defined by morbidity counts (1-2 low, 3-4 medium,> 5 high) and by a measure of severity of individual morbidities based on chronicity. Associations between comorbidity and physical function were assessed using unconditional logistic regression, adjusting for age, sex, and socioeconomic deprivation. RESULTS:Of the 1026 OA consulters, 38 (3.7%) had an OA consultation only, 260 (25.3%) had low, 288 (28.1%) medium, and 440 (42.9%) high morbidity counts. Higher OA comorbid counts were associated with poorer physical function, after adjusting for age, sex, and socioeconomic deprivation. Associations between OA comorbidity severity and poor physical function showed estimates that were in excess of simply multiplying the individual effects of OA and comorbidity severity separately. Comorbidity, however, did not explain all of the association between OA and poor physical function. CONCLUSION:Comorbidity increases the likelihood of poor physical function in patients with OA in population-based family practice. The combined influence is greater than would be expected from the influence of either OA or the comorbid conditions alone. Treating comorbidity in patients with OA is likely to be crucial in preventing or reducing the related physical decline.