Collagen vascular diseases are multisystem disorder and one of their extra-articular manifestations is pulmonary involvement. Pulmonary diseases in systemic sclerosis (SSc) are very common and it is the leading cause of mortality and the principle source of morbidity. Also it is not a rare complication of rheumatoid arthritis (RA) and the mortality from which in RA is twice that of general populations. High resolution computed tomography (HRCT) - being a sensitive and a non-invasive method- was used in this study to assess lung involvement in SSc and RA patients and to correlate the findings with clinical and laboratory parameters, plain chest radiography and pulmonary function tests (PFT).This study compromised 21 SSc and 20 RA patients in addition to 20 healthy controls. HRCT revealed pulmonary abnormalities in 61.8% of SSc patients and in 55% of RA patients mainly in the form of interstitial fibrosis. In SSc group, there was significant association between lung involvement by HRCT with severity of Raynaud’s, digital ulceration, tendon friction rub, arthralgia and numbness of fingers. Also there was significant positive correlation between HRCT score and ages of the patients, age of onset of the disease. In RA group, there was significant association between lung involvement by HRCT and the presence of subcutaneous nodules, severe form of the disease but there was insignificant association with other clinical or laboratory parameters. As regard pulmonary function tests in SSc and RA patients there was significant negative correlations between HRCT score and forced vital capacity percent (FVC %) in both diseases and forced expiratory flow of 25- 75% of forced vital capacity (FEF25-75%) in RA only. Also, HRCT showed significant superiority over plain chest x-ray (CXR) and there was significant association between reticular patterns on CXR and pulmonary abnormalities by HRCT.