Protrusio acetabuli is common is rheumatoid arthritis. Progression of this deformity can be measured by serial roentgenograms. The inward progression of the femoral head is the result of upward migration of the acetabular roof and collapse of the femoral head which appears to occur in a ratio of approximately two to one. Adrenal cortical steroids may play a significant role in the etiology of this condition in rheumatoid arthritis. This complication of intrapelvic protrusion of the prosthetic acetabulum following total hip replacement has been presented and suggestions made to overcome this problem. Early operation on those patients showing rapid development of protrusio may prevent complications.