To report serial intravascular ultrasound (IVUS) findings of bifurcation lesions treated with the dedicated Tryton Side Branch Stent to assess mechanisms of restenosis. The Tryton FIM study was a multicentre, prospective, single-arm, "first-in-man" study to treat de novo bifurcation lesions. Minimum lumen area (MLA) sites and overall volumes were analysed within main vessels and side branches. Overall, 27 main vessels and 22 side branches had paired baseline and follow-up IVUS. The post-intervention main vessel MLA decreased from 5.3 (4.1, 6.2) to 4.8 (3.4, 5.7) mm2 at follow-up, p=0.02, and the side branch MLA decreased from 3.5 (3.0, 3.8) to 2.5 (2.2, 3.2) mm2, p=0.0005. Stent area at the side branch did not change (mean stent area from 4.0 [3.3, 4.1] to 3.8 [3.4, 4.2] mm3/mm, p=0.95). Neointimal hyperplasia (NIH) net volume obstruction (%) measured 1.8% (0.5, 7.0) for the entire main vessel and 14.9% (2.3, 31.1) for the entire side branch stents. In both main vessel and side branches the decrease in lumen area correlated significantly with NIH. Serial IVUS analysis of a new side branch Tryton stent showed no chronic stent recoil. Side branch underexpansion was common and along with superimposed NIH contributed to the reduction in lumen dimensions at follow-up.