Objectives: To evaluate the outcome of Müller's muscle-conjunctival resection (MMCR) for correction of upper eye-lid ptosis and to explore the relationship between phenylephrine testing, muscle resection, and surgical outcome. Methods: Medical records were reviewed for all patients who underwent MMCR at the Jules Stein Eye Institute, Los Angeles, California, from January 1, 1999, through June 30, 2005. Outcome measures were margin reflex distance-1, ptosis correction after instillation of phenylephrine drops, extent of MMCR, ptosis correction, and eyelid symmetry. Results: In 80 patients who underwent 131 MMCR procedures for correction of upper eyelid ptosis, margin reflex distance-1 increased on average by 1.6 mm (P < .001). In 106 patients (81%), eyelid symmetry equal to or less than 1 mm was achieved (P=.02). Phenylephrine testing underestimated the extent of ptosis correction achieved with MMCR. A weak correlation was found between the extent of MMCR and ptosis correction (r=0.2; P=.04). Conclusions: Müller's muscle-conjunctival resection is effective for ptosis correction in patients with good levator muscle function; good eyelid symmetry is achieved in most patients. Phenylephrine testing underestimated the ptosis correction achieved with MMCR by 40%. The relationship between MMCR and ptosis correction is complex.