Rehydration, i.e. adding water to the dryed stool specimen before development, is a simple procedure to increase the sensitivity of the Hemoccult test. To counteract an inevitable loss of specificity, a diet omitting red meat and peroxidase-rich vegetables was recommended. During one year the stools of 3430 outpatients of our clinic were tested parallel in the standard manner and after rehydration. Despite of the low-peroxidase diet, rehydration lead to an increase of the positivity rate from 3.5 to 8.4%. Two carcinomas and 12 large adenomas were detected additionally. The predictive value of a positive test for colorectal neoplasia dropped from 20.5 to 13.7%. However, because of the doubling of the positivity rate and consequently the costs of follow-up examinations rehydration cannot be recommended as an improvement in mass-screening for colorectal cancer.