Relation between quadriceps muscle mass and knee extension strength was investigated after resection of malignant bone tumor and endoprosthetic replacement of the distal femur in 19 patients with patellar resurfacing (Group A) and 16 patients without resurfacing (Group B). In all cases, a modular cementless, hinged prosthesis had been used. Age and follow-up period were comparable in the 2 groups. Muscle mass was measured ultrasonographically and strength was evaluated isokinetically. All the data on the operated side are given as percentage of the non-operated side. There was a positive correlation between muscle mass and strength in both groups. By linear regression analysis, a slightly lower value of strength was predicted in the unresurfaced group than in the resurfaced group. Analysis of covariance revealed that the influence of patellar resurfacing on strength was significant. This factor should also be considered in making a decision about patellar resurfacing with a tumor endoprosthesis.