Hypovolaemia after overnight fasting is believed to exacerbate postoperative nausea and vomiting (PONV). However, data on the efficacy of supplemental i.v. crystalloids for PONV prophylaxis are conflicting. We performed a literature search using CENTRAL, MEDLINE, EMBASE, CINAHL, and Web of Science. We included prospective randomized controlled trials that reported PONV event rates in patients receiving supplemental i.v. crystalloids or a conservative fluid regimen after elective surgery under general anaesthesia. Studies were evaluated with regard to random sequence generation, allocation concealment, blinding of participants, personnel, and outcome assessment, incomplete outcome data, and selective reporting. We identified 15 trials (n787 crystalloids; n783 conservative fluids). Compared with conservative fluids, i.v. crystalloids reduced the risk of early postoperative nausea (PON) (relative risk 0.73, 95 confidence interval 0.590.89; P0.003), late PON (0.41, 0.220.76; P0.004), and overall PON (0.66, 0.460.95; P0.02). I.V. crystalloids did not reduce the risk of early postoperative vomiting (POV) (0.66, 0.371.16; P0.16) or late POV (0.52, 0.251.11; P0.09), but did reduce overall POV (0.48, 0.290.79; P0.004). I.V. crystalloids did not reduce the risk of early PONV (0.74, 0.491.12; P0.16), but did reduce the risk of late PONV (0.27, 0.130.54; P0.001) and overall PONV (0.59, 0.420.84; P0.003). I.V. crystalloids reduced the need for antiemetic rescue treatment (0.56, 0.450.68; P0.001). In summary, supplemental i.v. crystalloids were associated with a lower incidence of several PONV outcomes. However, a number of PONV outcomes failed to reach statistical significance, perhaps due to the lack of power. Thus, studies sufficiently powered for the less frequent outcomes (e.g. POV) are required.