To compare the effectiveness of preoperative subconjunctival injection of Mitomycin-C at 24-hour and at one month before primary pterygium excision. Randomised controlled trial. Eye Department, Combined Military Hospital, Sargodha, from January to December 2014. Eyes of 60 patients were randomly allocated into two equal groups (A and B) of 30 each. In group A Mitomycin-C was injected into the pterygium one month before, and in group B Mitomycin-C was injected 24 hours before excision. All pterygia received 0.1 ml of Mitomycin-C in a concentration of 0.15 mg/ml. Primary pterygia greater or equal to 2.5 mm were selected and excised as a bare sclera technique. Postoperatively, all patients were followed-up for 06 months. The recurrence and corneoscleral complications were recorded. Pearson chi-square test was used to compare the recurrence between group A and group B. The recurrence of pterygium in group A was 3.3%; and in group B, it was 6.7%. Comparison of the recurrences between both the groups was statistically insignificant, (p=0.554).Postoperatively, no serious corneoscleral complications occurred in either group. The preoperative subconjunctival injection of Mitomycin-C in a dose of 0.15 mg/ml given 24 hours prior to excision is as effective as 01 month preoperative injection for primary pterygium surgery with bare sclera technique.