Background: Stenosing tenosynovitis is a condition due to a size mismatch between the flexor tendons and the first annular pulley. Corticosteroid injection is the mainstay treatment. The purpose of this study is to compare different dosages and volumes of triamcinolone in the treatment of primary stenosing tenosynovitis. Methods: Patients with primary Quinnell grades 1 or 2 stenosing tenosynovitis were recruited in this prospective, blinded, randomized trial. Patients were randomized into 1 of 2 groups. Group A received 0.25 mL of triamcinolone 40 mg/mL, mixed with 0.25 mL of 1% lidocaine with epinephrine (10 mg of triamcinolone, 0.5 mL in total volume). Group B received 0.5 mL of triamcinolone 40 mg/mL, mixed with 0.5 mL of 1% lidocaine with epinephrine (20 mg of triamcinolone, 1 mL in total volume). Patients were assessed by a blinded hand therapist at 2 and 4 weeks, and by a blinded hand surgeon at 6 weeks. The primary outcome was complete symptom resolution at 6 weeks. Both per-protocol and intention-to-treat analyses were performed. Results: One hundred ninety-one patients were recruited from 2009 to 2018. Eighty-two and 77 patients had complete data in group A and B, respectively. There was no difference in success rates in complete symptom resolution at 6 weeks between group A (59.8%) and group B (62.3%). The mean visual analogue pain scores on injection were 4.31 ± 2.11 for group A and 4.30 ± 2.09 for group B. Conclusions: Triamcinolone 10 mg was as effective as 20 mg in the resolution of symptoms of Quinnell grade 1 or 2 stenosing tenosynovitis at 6 weeks.