Reported incidence rates of carcinoma in patients with achalasia and the prevalence of achalasia in patients with esophageal cancer vary widely in the literature. The prognosis of an "achalasia-carcinoma" is generally considered poor, although systematic studies assessing the incidence, prevalence, and prognosis of patients with "achalasia-carcinoma" are scant. We investigated the incidence of esophageal cancer in a large series of patients with known achalasia, assessed the prevalence of achalasia in patients presenting with esophageal cancer, and evaluated the prognosis of these patients compared to that of patients with esophageal cancer without achalasia. Between 1982 and 1998 a total of 124 patients with primary achalasia were treated and followed at our department. During the same time period 1366 patients presented with esophageal cancer (879 esophageal squamous cell carcinomas, 487 adenocarcinomas). Of the 124 patients with primary achalasia, 4 developed a carcinoma during a mean follow-up of 5.6 years (i.e., an incidence of one carcinoma per 173.6 patient-years of follow-up). Altogether, 13 of 879 patients (1.5%) presenting with esophageal squamous cell carcinoma and 1 of 487 patients (0.2%), presenting with esophageal adenocarcinoma had a history of primary achalasia. Seven patients with achalasia-carcinoma (50%) had early-stage disease (stage I, IIA, or IIB). There was no difference in the prognosis of patients with resected achalasia-carcinoma versus those with esophageal carcinoma but no achalasia. Thus in our population of patients with long-standing achalasia the risk for developing an esophageal cancer was increased about 140-fold over that of the general population. With liberal use of surveillance, carcinoma could often be detected at an early stage in these patients, with a prognosis that was not worse than that of patients with squamous cell esophageal cancer but no achalasia.