OBJECTIVES: The aim of the study was to compare the influence of sleeve gastrectomy (SG) versus Roux-en-Y gastric bypass (RYGB) on liver function in bariatric patients with non-alcoholic fatty liver disease (NAFLD) in a randomized clinical trial (NCT01806506). BACKGROUND: Rapid weight loss and malabsorption after bariatric surgery in patients with NAFLD or steatohepatitis (NASH) may impair liver function. METHODS: Sixty-six morbidly obese patients randomized to SG or RYGB were included in a secondary outcome analysis. Intraoperative liver biopsies were categorized with NAFLD Activity Score (NAS) and liver function tests were done before surgery and after 1, 6 and 12 months. RESULTS: NASH was present in 54.5% RYGB and 51.5% SG patients (P > 0.05). At 12 months excess weight loss was 68.7 ±â€Š19.7% after SG and 62.8 ±â€Š18.5% after RYGB (P > 0.05). At 1 month international normalized ratio (INR) increased after RYGB (0.98 ±â€Š0.05 vs 1.14 ±â€Š0.11; P < 0.05) and SG (0.99 ±â€Š0.06 vs 1.04 ±â€Š0.06; P < 0.05), RYGB induced significantly greater increase in INR in the whole group and NASH patients than SG. After RYGB albumin decreased at 1 month (41.2 ±â€Š2.7 vs 39.0 ±â€Š3.2 g/L; P < 0.05). At 12 months, INR and albumin returned to baseline. At 12 months in NASH group, SG induced significant improvement in aspartate aminotransferase (32.4 ±â€Š17.4 vs 21.5 ±â€Š6.9U/L), alanine aminotransferase (39.9 ±â€Š28.6U/L vs 23.8 ±â€Š14.1U/L), gamma-glutamyl transpeptidase (34.3 ±â€Š16.6 vs 24.5 ±â€Š16.8U/L), and lactate dehydrogenase (510.8 ±â€Š33 vs 292.4 ±â€Š29). Variables predictive of INR change after 1 month included operation type, NAS ≥ 5, bilirubin, body mass index, hemoglobin A1C, and dyslipidemia. CONCLUSIONS: Patients with NASH undergoing RYGB are more susceptible to early transient deterioration of liver function than after SG.