OBJECTIVE To assess whether knowing blood cholesterol test results influences people's intention to lower their dietary fat intake and to assess changes in diet after 3 months. METHODS Randomized clinical study. METHODS Two hospital-based family medicine centres. METHODS A total of 526 patients aged 18 to 65, without prior knowledge of their blood cholesterol levels, were recruited. Seventy did not appear for their appointments, and 37 did not meet study criteria, leaving 419 participants. From that group, 391 completed the study. METHODS Patients submitted to cholesterol screening were randomly assigned to one of two groups, completing the study questionnaires either before (control group) or after (experimental group) being informed of their screening test results. All participants were called 3 months after transmission of test results to assess their dietary fat intake at that time. METHODS Differences in intention to adopt a low-fat diet reported between the experimental and control groups and differences in dietary fat intake modification after 3 months between patients with normal and abnormal blood cholesterol test results. RESULTS Knowledge of test results influenced patients' intentions to adopt low-fat diets (F1,417 = 5.4, P = .02). Patients reported lower mean dietary fat intake after 3 months than at baseline (P < .0001). The reduction was greater in patients with abnormal screening results (F2,388 = 3.6, P = .03). CONCLUSIONS Being informed of personal blood cholesterol levels effects an immediate change in eating habits that translates into reduced dietary fat intake.