The authors evaluated the prevalence of paraarticular ossifications in three groups of patients after a minimum interval of one year following the administration of a total prosthesis. For evaluation they used Brooker's classification. The first group comprised 40 patients who during the first six weeks after administration of a cemented prosthesis of the hip joint took 75 mg of Indomethacin per day in three doses. The second control group comprised 50 patients, i.e. 61 operated hip joints (11 bilateral prostheses) to whom Indomethacin was not administered during the postoperative period, nor any other antiphlogistic preparations. The third group comprised 40 patients to whom a non-cemented prosthesis of the hip joint was implanted and who did not use any antiphlogistic preparations after operation. In the first group, i.e. patients after total endoprostheses of the hip joint with preventive administration of Indomethacin, ectopic ossifications were recorded in 32.5% of the operated patients. In the second group, i.e. without preventive Indomethacin administration, ectopic ossifications of various grades were recorded in 51%. In patients with non-cemented prostheses of the hip joint the prevalence of ectopic ossifications was only 18%. The authors selected from the control group a sub-group with bilateral prostheses of the hip joint where an ectopic bone was found. This sub-group comprised 9 operated patients and bilateral ectopic ossifications developed only in 33.3%. From the results ensues that Indomethacin administration is an expedient prevention of development of paraarticular ossifications. Marked reduction of development of ectopic bone formation occurs when bone cement is not used as a fixation medium.(ABSTRACT TRUNCATED AT 250 WORDS)