Objectives : To asses the value of Power Doppler Ultrasonography (PDUS) in the detection of inflammatory activity of the knee joints of patients with rheumatoid arthritis and to correlate the results with various disease parameters. Patients and methods: 30 adult patients (27 females and 3 males) with rheumatoid arthritis were assessed by full history taking, complete clinical examination, routine laboratory investigations, plain radiography and power Doppler ultrasonography of both knee joints. Ten healthy adults of matching age and sex (9 females and 1 male) served as control group. Results: Results showed the following findings : There was a highly significant increase in the synovial thickness of the knees of RA patients compared to those of the control group (P=0.000). It was detected in 42 out of 60 examined knees (70%). There was significant increase in the degree of vascularity of the synovial membrane of the knees of RA patients compared to those of the control group (P=0.02). It was detected in 6 out of 60 knees examined (10%). It had highly significant positive correlation with modified index of synovitis activity (P = 0.000) and significant positive correlation with Chronic Arthritis Systemic Index (CASI) (P = 0.02). Erosions were found in 6 out of 60 examined knees (10%), they had highly significant positive correlation with duration of knee symptoms (P = 0.002). Clinical examination revealed 20 swollen knees (23.3%) of our RA patients but on examination by US, effusion was found in 18 out of these 20 knees (90%) and synovial thickening was the only finding in the other two (10%). On the contrary 40 knees (66.7%) were considered clinically free but on US examination minimal effusion was found in 22 knees (55%). Effusion detected in our RA patients showed no echogenic linear structures representing fibrous strands, synaechae or loose bodies. Effusion had highly significant positive correlation with modified index of synovitis activity (P = 0.000), knee score (P = 0.000) and had significant positive correlation with Ritchie Articular Index (RAI) (P = 0.01).- Ultrasound detected Baker's cysts in 13 out of 60 examined knees (21.7%), complicated Baker's cysts were found in 6 knees (10%) 4 of them had turbid fluid and the other 2 had been ruptured. The size of Baker's cyst had highly significant positive correlation with modified index of synovitis activity (P= 0.008), it had significant positive correlation with Ritchie Articular Index (RAI), intensity of pain and General Health Assessment (GHA) of patients using Visual Analouge Scale (VAS) (P = 0.02). Joint space narrowing due to thin menisci was found in 35 out of 60 examined knees (58.3%). Joint space narrowing had highly significant positive correlation with age of the patient (P= 0.000), Steinbrocker functional capacity (P = 0.004) and had significant positive correlation with General Health Assessment (GHA) of patients using Visual Analouge Scale (VAS) (P= 0.02). Thin cartilage of the examined knees was found in 11 out of 60 examined knees (18.3%). There was no significant decrease in the cartilage thickness of the knees of RA patients and the control group (P = 0.5). Meniscal tear and cyst was found in one knee (1.7%). None of our patients had torn ligaments. None of our controls had increased vascularity, effusion, Baker’s cyst, joint space narrowing, meniscal tear, erosions or subchondral bone cysts. Conclusion :We conclude from our study that Power Doppler Ultrasonography is reliable for assessing inflammatory activity of the knee joints as detection of increased vascularity is helpful to differentiate between active and inactive pannus, also PDUS is better in detection of effusion, Baker's cyst and synovial thickening than clinical examination. Also PDUS is much better in detection of erosions than conventional radiography and lacks the hazards of radiation.