Measurements of American Urological Association symptom score, peak urine flow rate and prostate specific antigen (PSA) are often followed over time in urological management. However, their interpretation is confounded by within-patient variability due to chance. Data from 2 clinical trials are used to examine the magnitude of this variation. When these measures are repeated at a short interval variation is modest and might easily be misinterpreted as a true change in patient condition. For example, approximately 20% of patients might be expected to have a chance increase or decrease in symptom score by at least 4.9 points, in peak urine flow rate by at least 4.1 ml. per second or in PSA by at least 1.6 ng./ml. Clinicians can use these data to help interpret repeated measures of these variables in patients, and can consider obtaining paired measurements to decrease the effect of chance variation when they plan on following them over time.