We studied the natural history of asymptomatic early avascular necrosis of the femoral head in forty-three patients(forty-three hips) who had been treated with arthroplasty due to progressed avascular necrosis of femoral head but had no symptoms in the other side. All initial lesions were classified with Ficat staging and International classification(ARCO). The duration until symptom onset and radiologic evidence of disease progression were evaluated. The average follow up period was over 37 months. Among forty-three hips, twenty-nine cases(67.4%) remained asymptomatic until last follow up but fourteen cases(32.6%) were symptomatic during the follow up, and twenty-six cases(60.5%) did not progress on the radiography but seventeen cases(39.5%) progressed to Ficat stage 3 or 4. Fifteen cases of IA, IIA, IB-cental and II B-central without lateral lesion did not occur symptom and didnt progress to Ficat stage 3 or 4 over a period of average 47 months(24-71 months). But fourteen cases of IB, IC, g B and II C progressed to Ficat stage 3 or 4 and they were all lateral lesion. These results suggest careful observation only can be done in the case of small involvement(less than 15%) and medial or central lesion relatively over a long period of time but for the large involvement(more than 30%) and especially lateral lesion, aggressive surgical treatment may be considered even though the early stage of disease for the prevention of progression. However it will need more period of time to accurately compare the results of various methods of treatment.