A prospective study on a sample of patients living in Alnajaf,related to osteoarthritis of the knee and some factors affecting it. The objectives of this study is to find the relation between age, sex and overweight to osteoarthritis of the knee joint. Between Nov. 2000 and Nov. 2002, all the patients attended the teaching hospital in AlNajaf, complaining of knee pain were clinicaly examined. Those who proved clincaly to have primary (idiopathic) osteoarthritis of the knee joint were included in this study. From those patients a sample of 126 patient were further investigated; The body mass index of each was calculated from their weights and hights. Bilateral weightbearing x-ray of the keens were taken to see the radiological features of osteoarthritis in details. Total number of patients was 730, 606 females and 124 males .The ratio of females to males was 4.8:1.The mean age of the females was 54.7 years and of the males was 60.6 years. For the sample of 126 patients the mean body mass index of the females was 30.5Kg/m2, and of the males was 27.5 Kg/m2. They were 61 obese, 55 overweight and 8 patient with accepted weight. Decrease joint space especially on the medial compartment was the most common radiological feature, followed by the sub-chondral sclerosis which occurred mainly on the medial compartment, and osteophytes occurred mainly on the lateral compartment. In conclusion: Idiopathic osteoarthritis of the knee occur more in women.Age and overweight has a direct effect on the development of primary osteoarthritis of the knee joint.l