BACKGROUND: Topical glyceryl trinitrate treatment has demonstrated short-term efficacy in chronic noninsertional Achilles tendinopathy. No long-term followup is reported. We aimed to assess if the demonstrated efficacy of this treatment persisted 3 years after discontinuation of therapy. METHODS: A follow-up study of 52 patients (68 tendons) treated with 6 months of glyceryl trinitrate therapy or placebo was performed 3 years after cessation of therapy. Assessment included pain scores, return to previous activity, the Victorian Institute of Sport Achilles tendon scale (VISA-A), asymptomatic patient outcomes, clinical assessment of tendon tenderness, and functional hop test. RESULTS: Patients treated with topical glyceryl trinitrate had significantly less Achilles tendon tenderness (p = 0.03), and improved VISA-A scores (p = 0.04) than those in the placebo group; 88% (28 of 32 tendons) of patients were completely asymptomatic at 3 years (VISA-A score of 100) compared to 67% (24 of 36 tendons) of patients treated with rehabilitation alone (p = 0.03 with Chi square analysis). Other outcome measures showed nonsignificant trends towards improvement in the glyceryl trinitrate group (pain scores p = 0.07, functional hop test p = 0.07, and return to sport p = 0.16). The mean estimated effect size for all outcome measures was 0.21. CONCLUSIONS: Topical glyceryl trinitrate treatment has demonstrated efficacy in treating chronic noninsertional Achilles tendinopathy, and the treatment benefits continue at 3 years. Significant differences in asymptomatic patient outcomes for the glyceryl trinitrate group continue at 3 years, and this is confirmed by the effect size estimate. This suggests that the mechanism of action of topical glyceryl trinitrate on chronic tendinopathies is more than an analgesic effect.