Pulmonary function tests and chest radiographs of 29 non-smoking systemic sclerosis (SSc) patients were analysed, featuring an apparently paradoxic finding of an increased diffusing lung capacity for carbon monoxide (DLCO). Twenty-one patients (72%) had abnormal pulmonary function, 11 of them had restrictive disease (38%), six (21%) had isolated DLCO increase, four (14%) had isolated DLCO reduction, while two patients had obstructive disease (7%). Chest X-ray revealed interstitial abnormalities consistent with pulmonary fibrosis in all four patients with isolated DLCO reduction, in one obstructive patient and in six restrictive patients. In patients with DLCO increased steroid treatment significantly reduced DLCO (P < 0.05) and membrane DLCO component (Dm) (P < 0.05). Hitherto unobserved finding of DLCO increase in SSc patients was associated with shorter duration of SSc (P < 0.05), normal lung mechanics and roentgenogram (P < 0.05) and absence of pulmonary symptoms (P < 0.05). The findings that in some SSc patients DLCO increases suggest that DLCO might prove to be an early and sensitive indicator of acute pulmonary involvement.