The objectives of this study were to evaluate the primal cut composition of South African lamb carcasses with different fat scores, and to identify cuts suitable for fat trimming. Sixty grain fed Dorper lambs (rams and ewes) were divided into three groups and slaughtered at 30, 36 and 42 kg. Chilled carcass sides were subdivided into seven primal cuts. The cuts were dissected into meat (muscle, intermuscular and intramuscular fat), bone and subcutaneous fat (SCF) in order to determine the physical composition per cut and for the whole carcass. The % subcutaneous fat (SCF) in the loin increased the most (26 units) as the fat score increased from 1 to 5, followed by the flank, shoulder and neck. The % meat (lean) of the neck, thick rib and breast showed no significant change between fat scores 1 to 5, while % bone decreased significantly (>6% units). Meat and bone proportions decreased significantly with an increase in fat score for the loin, flank, leg and shoulder. The composition of the loin cut was most affected overall by changes in the fat score. Since different cuts accumulate SCF at different rates during fattening, trimming of SCF could reduce the boneless % SCF level of the major cuts such as the loin, leg and shoulder by 12, 6 and 9 units, respectively, when trimmed from SCF equal to a fat score 5 to a fat score 3. Further trimming to levels equal to a fat score 1, could reduce the % SCF by 18, 8 and 5 units, respectively on a boneless level. Considering differences in relative increase in intermuscular fat (IMF) and SCF in different cuts, the leg seems to be the most suitable cut for trimming in fat carcasses, since the IMF : SCF ratio is the lowest compared to the other larger cuts namely the loin and shoulder.