We screened a sample of 204 children between the age of 3 months to 5 years and 3 months were during a general health survey conducted by the Department of Community Medicine, Kasturba Medical College, Mangalore from March 1992 to December 1993 at Kumpala village under PHC Ullal about 10 kms from Mangalore City. We developed a new index RJ as RJ = Mid upper Arm Circumference x (Height)2 x 10(-4) and tested the correlation of RJ index and weight for age. We found the correlation coefficient (r = 0.8) to be very significant (P < 0.001). Thus we propose the model as Weight = A x exp (RJ x B). We found that the available data fit in this model. We hope that this study will be a new step in looking at arm circumference as a tool in estimating weight of pre-school children.