BACKGROUND The purpose of this study was to clarify the clinico-radio-pathological characteristics and prognostic factors of unclassifiable-idiopathic interstitial pneumonias (U-IIPs) diagnosed by surgical lung biopsy. METHODS Among 86 patients with interstitial pneumonia who underwent surgical lung biopsy from January 2005 to September 2013, 33 (38.4%; 16 male patients; mean age, 64.4 ± 8.8 years) were diagnosed with U-IIPs. They were subsequently categorized into rapidly progressive (n = 7), slowly progressive (n = 7), and stable (n = 19) groups based on the decrease of the percent predicted forced vital capacity or percent predicted diffusing capacity of the lung carbon monoxide and the occurrence of acute exacerbation. The clinico-radio-pathological features and survival rates of the patients who were followed up for at least 3 years were examined. These cases were reevaluated retrospectively by multidisciplinary discussion. RESULTS The rapidly progressive group had a significantly poorer prognosis than that of the other groups (p < 0.0001). Although there were no significant pattern differences on the chest high-resolution computed tomography, the fibrosis scores were significantly higher in the rapidly progressive group (p = 0.002). Furthermore, the percentage of fibroblastic foci assessed by the pathological analysis was also significantly higher in the rapidly progressive group (p = 0.006). Nine (27.3%) patients developed connective tissue diseases during follow-up. CONCLUSIONS The radiologic patterns were not significantly different among the three clinical U-IIPs subgroups. Nevertheless, our findings suggested that the fibrosis scores and the percentage of fibroblastic foci could provide a prognostic assessment in U-IIPs.