Uncertainty exists over whether to consider "lone" idiopathic pulmonary fibrosis (LIPF) and pulmonary fibrosis associated with connective tissue disorders (PFCTD) as significantly different entities. We retrospectively analysed data collected at the time of first diagnosis in 17 patients with LIPF and in 14 patients with PFCTD and compared survival in the two groups. At first evaluation, the time from onset of respiratory symptoms, spirometric volumes and the diffusing capacity for carbon monoxide were not significantly different between the two groups. However, arterial oxygen tension was significantly lower in LIPF than in PFCTD (63 +/- 3 vs 88 +/- 3 mmHg, p < 0.001). The radiological profusion scores relative to the upper and middle lung fields were significantly higher in LIPF than in PFCTD (upper regions: 6.9 +/- 0.6 vs 3.4 +/- 0.6, p < 0.005 - middle regions: 7.1 +/- 0.5 vs 4.8 +/- 0.7, p < 0.025), whereas the scores relative to the lower fields were similar (7.4 +/- 0.4 in LIPF and 8.4 +/- 0.6 in PFCTD). Survival since onset of respiratory symptoms was significantly better in the PFCTD than in LIPF patients, with a hazard ratio of 4.16 (95% CI 1.12-15.58, p=0.034). Thus, in our series of patients, those with LIPF had a more severe disease than those with PFCTD as shown by the higher frequency of hypoxaemia, the more diffuse pulmonary involvement demonstrated by the chest X-ray and the decreased survival.