To estimate the prevalence of low birth weight (LBW) and to document distribution of body mass index (BMI) at birth in Arak (central Iran) neonates of the 10,241 live neonates (5241 boys, 5000 girls, sex ratio 105) born in 2004 in Arak. A birth weight of less than 2500 g was classified as LBW. BMI based on the original supine length and weight data was calculated and compared with BMI at birth of Iran reference data. The overall prevalence of LBW was calculated as nine per cent, less pronounced among boys than girls. Over two-thirds of Arak neonates enjoyed normal weight and some five per cent of them were overweight or obese. However, one-quarter of neonates were classified underweight. Girls' BMI centiles lie below those of boys. Arak neonates were relatively free of obesity. However, the rate of neonatal underweight was striking. Neonatal LBW was more prevalent than the developed world. While LBW is a crude index, underweight BMI class is an adjusted index, which should be taken into consideration when one studies neonatal weight. (Contains 2 tables.)