In 2007 the American Academy of Sleep Medicine (AASM) published polysomnography (PSG) scoring guidelines, which were updated in 2012. A key change in terms of scoring respiratory events in children was the threshold for reduction in airflow (50 vs 30 ) for the definition of hypopnea. This study aimed to determine the impact of different scoring rules on the assessment of severity of obstructive sleep apnea (OSA) in children. METHODS: Forty-two children (mean age 4.3 y, 16 F) underwent PSG. An obstructive apnea-hypopnea index (OAHI) was determined using three scoring rules: (1) ATS 1996 rules with minor modifications (modified ATS 1996); (2) AASM 2007 rules (AASM 2007); and (3) AASM 2007 rules with respiratory event related arousals included in the OAHI (AASM+RERA). RESULTS: The AASM 2007 OAHI (median 0.4 events/h, range 0, 14) was lower than the modified ATS 1996 OAHI (median 0.8 range 0, 26.1, p <0.001), underestimating severity of disease in 24 of cases. The AASM+RERA OAHI (median 0.8, range 0, 19.1) was also lower than the modified ATS 1996 OAHI (p = 0.02), but the difference was not clinically significant except at very high OAHIs. CONCLUSION: The AASM 2007 rules lead to a lower OAHI and lesser OSA severity when compared to the previous standard. Inclusion of RERAs in the AASM 2007 OAHI leads to a comparable OAHI to the previous rules. Given that morbidity has been demonstrated even in mild OSA, these results support the inclusion of events with a reduction in airflow of less than 50 as included in the updated AASM rules in 2012.