The pulsed slow-positron beam technique was applied to study the structural changes in ultra-high molecular-weight polyethylene (PE) after carbon implantation at three implantation doses. The influence of subsequent gamma-irradiation on the structure of the carbon-implanted PE was studied. A three-layer model was used in order for the experimental results to be explained. An increase in the number of free-volume holes and a decrease in their sizes were observed for the subsurface layer. A considerable inhibition of the Ps formation in a layer next to the subsurface layer was detected. The position of the former, at the lowest carbon concentration, was in good agreement with the position of the carbon ions stopping region, calculated by the TRIM code. In the case of higher carbon concentrations significant discrepancy of the positions was observed.