The World Health Organization (WHO) recently updated the diagnostic criteria for serrated polyposis syndrome (SPS). One of the three previous diagnostic criteria (criterion II2010) is now abandoned: ≥ 1 serrated polyp (SP) proximal to the sigmoid in a first-degree relative (FDR) of a patient with SPS. Individuals fulfilling this abandoned criterion now receive the same surveillance recommendations as all FDRs of patients with SPS. We aimed to compare the incidence of advanced neoplasia (AN) in FDRs with vs. without fulfillment of the abandoned criterion II2010. We retrospectively recruited FDRs of patients with SPS who underwent a colonoscopy, and stratified them according to fulfilment of criterion II2010 at baseline. Our primary and secondary outcomes were AN incidence during surveillance and at baseline, respectively. We included 224 FDRs of patients with SPS, of whom 36 (16%) fulfilled criterion II2010 at baseline. One hundred and five underwent surveillance after baseline. Criterion II2010-positive FDRs were at increased risk of AN, both during surveillance (hazard ratio 8.94, 95% CI 2.15-37.1, p = .003) as well as at baseline (adjusted odds-ratio 9.30, 95% CI 3.7-23.3, p < .001). FDRs of patients with SPS that underwent colonoscopy and fulfilled the abandoned criterion II2010 for SPS diagnosis were at increased risk of AN at baseline and during surveillance in this small, retrospective cohort study. Our results should be interpreted with caution but suggest that adherence to surveillance recommendations for all FDRs of patients with SPS is important, especially for those that would have fulfilled the now abandoned criterion II2010.