In clinical trials that study people with a continuous measure defined categorically, even repeated measurements within a visit and over successive visits do not prevent error-free classification. We describe the design of a screening procedure for the Systolic Hypertension in the Elderly Program (SHEP), a randomized clinical trial designed to test whether regular administration of antihypertensive medication reduces the risk of stroke in elderly persons with isolated systolic hypertension. Data from a pilot study performed before the inauguration of SHEP allowed empirical study of a variety of possible screening rules for SHEP. A desirable screening rule would require only two screening visits, would lead to a randomized cohort with high mean systolic blood pressure, and would not impede recruitment. We emphasize two classes of rules, "serial" and "conditional." A serial rule uses only the values observed at a given screen to determine eligibility to proceed to the next screen. A conditional rule uses the value observed at a given screen along with values already observed to determine eligibility to proceed. For the SHEP study, we chose a conditional rule for screening because of its efficiency in identifying eligible participants. Our approach to selection of screening rules should be applicable to other clinical trials in which the measurement that defines the primary entry criterion has considerable measurement error.