Abstract: Since Nitric Oxide (NO) is produced by three types of Nitric Oxide Synthases (NOSs), rapid changes in stable oxidized metabolites (nitrite and nitrate, NOx) in the tissues and blood should be represented by the amount of stable forms in serum and may reflect vascular activities and circulatory or inflammatory changes in the body. Therefore, serum NOx levels in patients with various diseases were measured and compared to healthy controls. Four hundred and sixty five in- and outpatients aged 14 to 96 years were included in this study and 49 healthy hospital workers were included in the control group. The NOx levels of both groups were measured at rest in the morning using an ozone chemiluminescence method. When compared with the control group, serum NOx levels were higher in patients consist of around 40 or more in numbers diagnosed with cardiovascular diseases including myocardial infarction, hyperlipaemia, gastrointestinal diseases including acute enteritis, chronic liver diseases including viral B and C type hepatitis and liver cirrhosis, diabetes in males, hypertension with comorbid diseases. In addition, patient groups with renal disorders, hyperuricemia, osteoporosis, untreated cancers, autoimmune diseases such as Rheumatoid Arthritis (RA) and Systemic Lupus Erythematosus (SLE) were also higher, although the each number might be not enough to compare between the groups. Patient groups with acute and chronic inflammation had significantly higher serum NOx levels. Therefore, measurement of serum NOx in patients may be useful for understanding the status and pathophysiology of inflammatory diseases and those in which inflammation is a component.