The leptin receptor gene (LEPR) is a candidate for traits related to growth and body composition, and is located on SSC6 in a region where fatness and meat composition quantitative trait loci (QTL) have previously been detected in several F 2 experimental designs. The aims of this work were: (i) to fine map these QTL on a larger sample of animals and generations (F 3 and backcross) of an Iberian x Landrace intercross and (ii) to examine the effects of LEPR alleles on body composition traits. Eleven single nucleotide polymorphisms (SNPs) were detected by sequencing LEPR coding regions in Iberian and Landrace pig samples. Three missense polymorphisms were genotyped by pyrosequencing in 33 F 0 , 70 F 1 , 418 F 2 , 86 F 3 and 128 individuals coming from the backcross of four F 2 males with 24 Landrace females. Thirteen microsatellites and one SNP were also genotyped. Traits analysed were: backfat thickness at different locations (BF T ), intramuscular fat percentage (IMF P ), eye muscle area (EM A ), loin depth (LO D ), weight of shoulder (SH W ), weight of ribs (RIB W ) and weight of belly bacon (BB W ). Different statistical models were applied in order to evaluate the number and effects of QTL on chromosome 6 and the possible causality of the LEPR gene variants with respect to the QTL. The results support the presence of two QTL on SSC6. One, at position 60-100 cM, affects BF T and RIB W . The other and more significant maps in a narrow region (130-132 cM) and affects BF T , IMF P , EM A , LO D , SH W , RIB W and BB W . Results also support the association between LEPR alleles and BF T traits. The possible functional implications of the analysed polymorphisms are considered. © 2005 Cambridge University Press.