Objective.: Little is known about the presence of chronic morbidity in inflammatory arthritis (IA) patients at disease onset. Previous studies have been mainly performed in established IA patients or they focus on isolated co-morbid diseases. Our aim was to determine the prevalence of chronic diseases at the onset of IA and to determine whether this is different from the number that one might expect based on age and sex. Patients and methods.: A nested case-control study from 2001 to 2010 using data from patient electronic medical records in general practice. Totally, 3354 patients with newly diagnosed IA were included. Each patient was matched on age, sex and general practice with two control patients. In total, 121 different chronic diseases were studied. Results.: In total, 70% of the IA patients had at least one chronic disease at the onset of IA, compared with 59% of the control patients (P < 0.001). The highest prevalence in IA patients was found for cardiovascular diseases (35%), musculoskeletal diseases (27%) and neurological diseases (22%). Compared with the control patients, patients with IA had the highest increased risk for musculoskeletal diseases [odds ratio, OR = 1.7 (95% confidence interval: 1.6-19)] and for neurological diseases [OR = 1.6 (1.4-1.7)] at the onset of IA. Conclusion.: At the onset of IA, nearly three-quarters of patients with IA had at least one other chronic disease. Since multi-morbidity affects treatment and outcome of the IA patient, these diseases should be taken into account when treating IA patients. © The Author 2013. Published by Oxford University Press. All rights reserved.