In three patients, aged 79, 58 and 59 years, respectively, iron-deficiency anaemia (IDA) was diagnosed. All three had a right-sided colonic adenocarcinoma. In the first patient, a cardiologic cause was looked for instead of a simple laboratory investigation of the anaemia. The second patient received iron supplementation without sufficient diagnostic evaluation, and in the third patient the abdominal X-ray was inadequate for evaluation of the ascending colon; moreover, she responded well to iron supplementation therapy. IDA is a common problem in clinical practice that may have various causes. In the Netherlands, gastrointestinal bleeding is the major cause of IDA in men and women over 50 years of age. The three patients described illustrate that IDA should be considered a clinical sign. An accurate evaluation of the IDA to detect a convincing explanation is therefore necessary. Various causes can be found on duodenoscopy and colonoscopy, e.g. neoplasms, ulcers, angiodysplasia or polyps. Carcinoma of the ascending colon is a frequent cause of IDA, especially in those over 50 years of age and without upper gastrointestinal symptoms. Evaluation to exclude a right-sided carcinoma of the colon has a high priority in these cases.