BackgroundThe aim of this randomized split-mouth study was to assess the influence of primary flap position on the amount of coronal soft tissue regrowth and keratinized tissue (KT) 6 months after osseous resective surgery with fiber retention technique (FibReORS).Materials and methodsTwo contralateral posterior sextants in 16 patients were treated with FibReORS and randomly assigned to flap positioning either 2 mm below the bone crest (apical group) or at the level of bone crest (crestal group). Clinical parameters were recorded at 1, 3 and 6 months and patient-related outcomes during the first two post-operative weeks.ResultsHealing period was uneventful. Patient’s discomfort was similar in both groups. The overall soft tissue rebound was higher in the apical than in the crestal group (2.0 ± 1.3 mmversus1.3 ± 0.7 mm), but the difference was statistically significant only interproximally (2.2 ± 1.3 mmversus1.6 ± 0.8 mm). Multilevel analyses showed higher soft tissue rebound in sites with normal compared to thin phenotype (1.5 mm,p< 0.0001) and treated with flap positioned 2 mm apically to the bone crest (0.7 mm,p< 0.001). An additional 0.5 mm KT increase was observed at interdental sites in the apical group.ConclusionsApical flap positioning increases soft tissue rebound and KT width, mainly at the interdental sites, with reduced patient discomfort.Trial registrationThe trial was registered at ClinicalTrials.gov (ID: NCT05140681, Registration date: 1/12/2021, retrospectively registered).Supplementary InformationThe online version contains supplementary material available at 10.1186/s12903-023-03144-2.