Objective: To explore the role of the obese protein (OP) of the obese gene expressive product in the genesis and development of noninsulin-dependent diabetes mellitus (NIDDM). Methods: Plasma OP levels were measured by radioimmunoassay in 21 cases of normal subjects, 24 adult obese patients and 20 patients with NIDDM. At the same time, the other parameters were measured, including body mass index, plasma cholesterol, triglyceride, fast blood glucose, hemoglobin A1 and the plasma insulin level during glucose tolerance test. Results: The levels of the plasma OP in NIDDM patients (81.0±17.5) pg/ml were very significantly lower than those in normal subjects (194.3±17.7) pg/ml (P<0.01). The levels of the plasma OP in non-obese NIDDM patients were very significantly lower than those in non-obese normal subjects (P<0.01) and the levels of the plasma OP in obese NIDDM patients were very significantly lower than those in obese patients (P<0.01). Conclusion: The plasma OP may be closely related to the genesis of NIDDM. It is one of the important factors resulting in obesity that the plasma levels of OP were lower.