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Purpose To report clinical outcomes of keratoconus patients undergoing the sub-400 protocol in a UK tertiary referral centre.Methods We conducted a prospective audit of patients with progressive keratoconus and thin corneas (<400 µm) referred to Cornea Service at Royal Victoria Infirmary, Newcastle upon Tyne. Consecutive cases treated between March 2022 and June 2024 were included. The protocol involved epithelial debridement, 20 minutes of isotonic riboflavin application, pachymetry measurement at the thinnest point, and 3 mW/cm 2 UV exposure according to the collagen crosslinking (CxL) sub-400 protocol. Outcomes included demographics, BCVA (logMAR), slit-lamp examination, specular microscopy, and corneal tomography, with follow-up at 1, 3, 6, 12, and 18 months.Results Twelve patients (13 eyes; M:F ratio 5:7) with a mean age of 26.9 years (SD 7.2) were included. Preoperative mean visual acuity was 0.74 logMAR (SD 0.43), which remained stable at 6 months. Mild stromal haze was seen in 30.7% at 1 month, but resolved by 3 month visit. No other complications occurred. At 6 months, tomographic analysis showed mean K1 decreased from 58.07D to 57.45D, K2 from 64.49D to 64.05D, and Kmax from 77.34D to 76.06D, none statistically significant (p>0.05). Only 2 patients (15.4%) showed signs of progression. Pachymetry decreased from 386.2 µm to 364.6 µm (p=0.09). Endothelial cell counts remained stable.Conclusions Individual analysis showed that 84.6% (n=11) of eyes remained stable with no complications, suggesting the sub-400 protocol is a safe and effective option for halting keratoconus progression in thin corneas. Larger numbers with longer follow-up are warranted.