The incidence and latency period of collagen vascular disease (CVD) were surveyed prospectively in patients originally diagnosed as idiopathic interstitial pneumonia (IIP). We also examined whether there were differences between IIP and CVD preceded by interstitial pneumonia. Background information, extrapulmonary symptoms, smoking history, laboratory findings, prognosis (Kaplan-Meier estimates), respiratory functions, and radiological findings were compared. The subjects consisted of 68 patients of whom 13 (19%) developed CVD (RA; 5 cases, DMPM; 5 cases, SLE; 1 case, Sjoegren syndrome; 1 case, Overlap syndrome; 1 case). The latency period for development of CVD was 24.9 +/- 39.2 (mean +/- SD) months. IIP patients who developed CVD were predominantly female and were younger than those without CVD (p less than 0.05). These patients also had faster ESR, higher CPK values, and a higher incidence of arthralgia, joint deformity and clubbed fingers (p less than 0.05). Abnormal shadows around costo-phrenic angles were seen more frequently in patients with CVD (p less than 0.05). These results suggest that quite a number of patients with IIP develop CVD. The clinical course of these patients should be followed by keeping these clinical findings in mind.