Abstract: Intravenous Patient Controlled Analgesia (PCA) with morphine is often used for postoperative analgesia in morbidly obese patients. However, the required doses may increase postoperative respiratory disorders. Adjunction of small doses of ketamine could reduce dose and related side effects. Eighty morbidly obese patients undergoing upper abdominal surgery were randomly assigned to receive (during the postoperative period) either intravenous morphine 1 mg mL-1 (morphine group: M group) or morphine with ketamine 1 mg mL-1 for each (morphine ketamine group: MK group). Morphine consumption was evaluated by cumulative doses every 8 h for 48 h together with Visual Analogue Scale (VAS) scores at rest and at mobilization. Forced Vital Capacity (FVC) and Forced Expiratory Volume in first second (FEV1) were assessed by spirometric evaluation 24 and 48 h after initiation of PCA. Incidence of adverse effects was reported. cumulative morphine consumption was significantly reduced in morphine Ketamine (MK) group. It was 84 (9) mg vs 66(6) mg 48 h after PCA (p<0.05). VAS in both groups were similar at rest and at mobilization. Compared with preoperative values, (FVC) was significantly reduced in M group than MK group: 46% (8) vs 62% (9) (p< 0.05) at 24 h and 58 (7) vs 77% (11) (p<0.05) at 48 h (FEV1) was 60% (9) vs 81% (7) (p<0.05) at 24 h and 62% (6) vs 87% (11) (p<0.05) at 48 h after PCA initiation. PaO2, respiratory rate, and SPO2 were significantly lower in M group than MK group 32 h after PCA initiation (p<0.05). Incidence of nausea and desaturation were significantly higher in M group than MK group (p<0.05). Adding small doses of ketamine to morphine for PCA decreased cumulative morphine consumption, improved postoperative respiratory function and was associated with less adverse effects in morbidly obese patients undergoing upper abdominal surgery.