Familial cases of interstitial pneumonia (FIP) have been reported to be linked to mutations of the surfactant protein C (SP-C) gene. Based on this knowledge, we evaluated the characteristics of patients with adult-onset FIP in the Tokyo area using clinical and radiopathological findings, and further evaluated the genetic background of patients with FIP compared with sporadic IP patients using genetic sequencing of the SP-C gene. A total of 22 patients with FIP from 13 families were identified, and the mean age at first diagnosis of these patients was 50 +/- 2.7 years (range: 20-66 years). Based on the specimen histology, UIP and non-specific interstitial pneumonia accounted for 64 and 36%, respectively. Distribution of the interstitial pattern in HRCT imaging resulted in 36% upper lung dominant, 5% whole lung and 59% lower lung dominant. Two missense mutations in exon 4 (N138T) and exon 5 (N186S) were identified in the SP-C gene from 11 cases with FIP. Each exonic mutation consisted of DNA polymorphism. The frequencies of these DNA polymorphisms were evaluated among 11 subjects with FIP, 30 subjects with sporadic IP, and 43 healthy volunteers as controls. Interestingly, the genotype and allele frequencies in exon 5 were statistically different among these groups. In particular, the N186S substitution of exon 5 in the SP-C gene was shown in patients with FIP or sporadic IP, with a statistically higher frequency. While pathophysiological mechanisms remain to be elucidated, the N186S missense variant may have potential susceptibility in the development of IP. The gene or genes that prove to be important in the development of FIP may provide insights into the pathogenesis of other forms of interstitial lung diseases.