Prosthetic material was used in the repair of coarctation of the aorta in 72 patients followed for 8 months to 24 years after operation. False aneurysms occurred late postoperatively in 1 of 17 instances when a tube graft was used to replace the resected aorta and in 1 of 2 patients when the graft bypassed the obstructed area. A Dacron onlay patch graft (DOPG) was used in 52 patients as the primary method of repair, to revise a previous repair or as an adjunct to another procedure at the time of initial operation. True or false late aneurysm occurred between 6 and 18 years postoperatively in 5 of the 38 patients in whom a DOPG was used for primary repair of the coarctation or to revise an earlier repair, an actuarial incidence of 38% at 14 years. Diagnosis of aneurysm formation late postoperatively depends on awareness of this complication and careful assessment of lateral as well as posteroanterior chest radiographs. DOPG should be avoided whenever possible.