Does phenylbutazone exert an inhibitory influence on the progression of ossification of the lumbar vertebral column? In a retrospective study of 40 definite cases of ankylosing spondylitis the case histories were divided into periods demarcated by the radiological examinations. An adequately exact method was used for quantitative assessment of ossification. The periods were divided into three groups: (A) continuous phenylbutazone medication; (B) phenylbutazone medication, but not throughout the period; (C) no phenylbutazone medication. Rapid progression of ossification had occurred in group C. In group A, ossification had either remained absent or, if already in progress, had been arrested or substantially delayed. In early or relatively early stages of ankylosing spondylitis, continuous phenylbutazone medication can completely or largely control ossification of the vertebral column. Possibilities of further improvement of this therapy are discussed.