Lambs of Yankasa sheep were evaluated for effects of genetic and environmental factors on their body weights at birth, 3,6,9 and 12 months of age. The lambs were born between 1981 and 1985 in an on-going accelerated lambing programme at National Animal Production Research Institute (NAPRl), Nigeria, Type of birth significantly affected weights from birth to 12 months of age. Lambs born as singles were consistently heavier at all stages with the difference being most pronounced at weaning. Ram lambs were 0.22kg and 4.88kg heavier than ewe lambs at birth and 12 months of age, respectively. The effect of parity of ewe on lamb growth was significant at birth, 3 and 6 months of age. Lambs born during the rainy season had higher mean weights at birth and weaning. The order was however reversed at 6 months with lambs born during the dry season being heavier, though the difference was not significant. Effect of season was therefore significant at birth, 3 and 12 months of age only. Lambs born in 1981 had the heaviest mean weights from birth to 6 months of age. The superiority of the lambs sired by the fourth sire-group was maintained from weaning to one year. Additive adjustment of lamb weights for the identified environmental factors was prefered to multiplicative adjustments because of larger reduction of error variances associated with these factors. Estimates of repeatability for weights were 0.14, 0.06, 0.19, 0.28 and 0.17 at birth, 3, 6, 9 and 12 months respectively. Corresponding estimates for heritability were 0,22, 1.14, 1.06, 0.94 and C.51. Genetic and phenotypic correlations between birth weight and weaning weight, weaning weight and 6-month weight, and weaning weight and 9-month weight were found to be 0.10 and 0.17, 0.27 and 0.43, and 0.17 and 0.39, respectively. It was concluded that under this environment, it would be necessary to adjust body weights of growing lambs for the effects of type of birth, sex of lambs, parity of ewe, season and year of lambing