Objective To identify distinguishing characteristics between patients with idiopathic pulmonary fibrosis (IPF) and primary lung carcinoma and patients with either IPF or carcinoma alone. Patients and Methods The study group consisted of 24 patients with histologically proven usual interstitial pneumonia and lung carcinoma identified through a search of the Rochester Mayo Clinic database for 1990 to 1998. Medical records, radiographs, and histological slides were reviewed. Several variables including survival were compared in 2 control groups, IPF only and carcinoma only, by using various statistical methods. Results Our study group included 21 men and 3 women (mean age, 72.3 years). Twenty-two were past or current smokers. Approximately half of the lung carcinomas were incidental findings. Of the 14 patients with preoperative computed tomographic scans, 12 had peripheral tumors situated in areas of fibrosis. Squamous cell carcinoma was the most common histological type, accounting for 16 cases. Almost all patients underwent surgical treatment; nearly 40% developed postoperative complications, and 3 died within 30 days of surgery. The ratio of men to women in patients with IPF and carcinoma was 7:1 compared with 1:1 in patients with IPF only ( P=.003). Patients with IPF and carcinoma were also older, with a mean age of 72.3 years compared with 64.4 years ( P=.001), and were more often smokers ( P=.002). Carcinomas involved the lower lobes in 42% of patients with IPF and carcinoma compared with 29% of patients with carcinoma only ( P=.004) and were mainly composed of squamous cell carcinoma ( P=.004). Mean survival in patients with IPF and lung carcinoma was 2.3 years after the diagnosis of IPF and 1.6 years after that of carcinoma. This finding did not differ significantly from survival of patients with either IPF or carcinoma alone. However, statistical power was limited. Conclusion Carcinoma in patients with IPF arises in older male smokers and usually presents as peripheral squamous cell carcinoma. The prognosis is poor.