Progressive systemic sclerosis (PSS) is a generalized collagen disease which has oral manifestations, including widening of the periodontal ligament (PDL) space. The purpose of this study was to measure the extent of thickening of the PDL space in a group of 26 patients with PSS and in a similar group of age and sex-matched control patients. The width of the PDL was measured from periapical radiographs at four sites around seven selected teeth for every patient. The mean PDL width was found to be larger in all the patients with PSS than in the control patients, around both anterior and posterior teeth. One third of the teeth of patients with PSS had PDL widths thicker than the range observed in the control patients. When average thickness of PDL widths for each patient is compared, at least 65% of the patients with PSS have mean PDL widths which are greater than the range for control patients. The increased thickness of the PDL space associated with PSS is more pronounced in posterior teeth than in anterior teeth.