Full tendon width vertical transposition (Knapp procedure) of the horizontal recti is an established treatment for double elevator palsy (DEP) but the long-term stability of the surgical results have not been well studied. We undertook a retrospective study to determine the overall effectiveness of the Knapp procedure, the postoperative stability of alignment, and the influence of prior inferior rectus muscle recession (IRc) on the magnitude of correction. Nineteen patients with DEP underwent a Knapp procedure. Eight were corrected to within 5 delta of orthophoria, six were undercorrected, and five were overcorrected by at least 5 delta after a mean follow-up of 3 years (to last visit or to further surgical intervention). The average vertical correction was 37.5 delta in patients who underwent a prior IRc compared with 21.1 delta in patients with no prior IRc (p = < 0.0017). Over and undercorrections were more likely to occur in patients with prior IRc. Postoperative drift was towards increased effect in all patients. The seven patients with long-term (> 36 months) follow-up demonstrated an increased magnitude of correction (average = 12.6 delta) over an average follow-up of 76 months. The Knapp procedure had an increasing effect over time but the amount of vertical correction did not correlate with the size of the preoperative vertical deviation and was less predictable when a prior IRc had been performed.