There are many different operations to correct involutional blepharoptosis (IB); however, the outcome of the corrective surgery is rather unpredictable, regardless of the procedure employed. A reasonably predictable outcome can be achieved with careful intraoperative evaluation of the condition, with measuring of the margin reflex distance-1 (MRD-1) in supine position of the patients. With these prepositions, we collected data that indicated that our approach can achieve a predictable outcome. This was a prospective study of 21 consecutive patients (8 men and 13 women) involving 42 eyelids with IB. IB was defined as an MRD-1 of <2 mm. All 21 patients were informed of the purposes of the study, and underwent levator aponeurosis advancement. The MRD-1 was measured intraoperatively with the patients in a supine position and in the 3-month postoperative inspection with the patients in a sitting position. Statistical analyses using paired t-tests were performed. From intraoperative measurement, mean MRD-1 values were 4.31 mm on the right side (range 3.0-4.5) and 4.29 mm on the left side (range 3.5-5.0). Three months after the operations, mean MRD-1 values were 3.07 mm on the right side (range 1.5-4.0) and 3.07 mm on the left side (range 2.0-4.0). Compared with the intraoperative MRD-1 measurements, those of the postoperatives were significantly 1.2 mm reduced (right: P < 0.01, left: P < 0.01). The intraoperative measurement of MRD-1 without changing position of patients could result in successful outcome of the operation.