OBJECTIVES: An obese body mass index (BMI) increases morbidities, however there are few chronic disease registries that can quantify the cost of obesity. Clalit Health Services (CHS), with complete longitudinal data of over 4 million members, provides an ideal format for comparing health care utilization between obese and non-obese patients to inform the need for improved health care policy. METHODS: For the years 2011-2013 inclusive, we took two random samples of 10,000 obese (BMI>30) patients and 10,000 normal and underweight (BMI<=25) from the CHS database both according to Clalit population age standard. We then extracted their additional chronic diseases from the CHS registry. Finally, we compared the average length of stay (LOS) for inpatient admissions between the groups, by disease. RESULTS:  Obese patients with underlying chronic disease had, on average, a 27% increased LOS compared to non-obese patients with chronic disease. The greatest effect was seen among obese patients with chronic renal failure, whose LOS was 2.7 times or nearly 20 days longer. By disease: ischemic heart disease, 1.9 times or 10 days longer; hypertension 1.3 times or 4 days longer; congestive heart failure, 1.2 times or 3 days longer; and rheumatoid arthritis, 1.4 times or 2 days longer.  Obese patients with diabetes and s/p cerebral vascular accident had a shorter LOS (0.8 times or 3 days, and 0.8 times or 4 days respectively). CONCLUSIONS: Obesity increases the LOS for all-cause hospital admissions among patients with various underlying chronic diseases. This may be due to insufficient diagnosis by the primary provider or specialist, inadequate medication dosing (eg, pain management), or inadequate support during an inpatient stay. A proactive health care policy is needed to guide the management of patients with chronic disease who are also obese, with the potential for cost-savings of interventional, pharmaceutical, or surgical treatment of obesity at baseline.