D-penicillamine has been used for the treatment of systemic sclerosis for 2 decades. Forty-four systemic sclerosis patients who received a mean dose of 636 mg of D-penicillamine for 2.3 years and 48 untreated patients, who had repeat pulmonary function tests performed after a mean of 3.5 and 4.8 years, respectively, were evaluated. There were no significant changes in the vital capacity or the forced expiratory volumes during this time in either group. However, the diffusing capacity for carbon monoxide improved from 76% of predicted to 87% of predicted in D-penicillamine-treated patients. Untreated patients changed slightly, from 73% of predicted to 76% of predicted. When multiple logistic regression analysis was used to account for factors which could have biased treatment comparisons, the improvement related to use of the D-penicillamine was significant (P less than 0.05). This improvement in the D-penicillamine-treated patients was associated with no further progression of dyspnea or fibrosis on chest radiograph, as well as reduced skin thickening. These data suggest that D-penicillamine may be useful in the treatment of systemic sclerosis that involves the lung; however, further studies are needed.