The anaemia seen in chronic kidney disease (CKD) may be exacerbated by iron deficiency. Iron can be provided through different routes, with advantages and drawbacks of each route. It remains unclear whether the potential harms and additional costs of intravenous (IV) compared with oral iron are justified. To determine the benefits and harms of IV iron supplementation compared with oral iron for anaemia in adults and children with CKD. In March 2010 we searched the Cochrane Renal Group's specialised register, the Cochrane Central Register of Controlled Trials (CENTRAL) in The Cochrane Library, MEDLINE and EMBASE without language restriction. We included randomised controlled trials (RCTs) and quasi-RCTs in which oral and IV routes of iron administration were compared in adults and children with CKD. Two authors independently assessed study eligibility, risk of bias, and extracted data. Results were reported as risk ratios (RR) or risk differences (RD) with 95% confidence intervals (CI) for dichotomous outcomes and for continuous outcomes the mean difference (MD) was used or standardised mean difference (SMD) if different scales had been used. Statistical analyses were performed using the random-effects model. Subgroup analysis and univariate meta-regression were performed to investigate between study differences. Twenty eight studies (2098 participants) were included. Risk of bias attributes were poorly performed and/or reported with low risk of bias reported in 12 (43%) studies for sequence generation, incomplete outcome reporting and selective outcome reporting and in 6 (16%) studies for allocation concealment. No study was blinded for participants, investigators and outcome assessors but all were considered at low risk of bias because the primary outcome of haemoglobin was a laboratory outcome and unlikely to be influenced by lack of blinding. Haemoglobin (22 studies, 1862 patients: MD 0.90 g/dL, 95% CI 0.44 to 1.37); ferritin (24 studies, 1751 patients: MD 243.25 μg/L, 95% CI 188.74 to 297.75); and transferrin saturation (18 studies, 1457 patients: MD 10.20%, 95% CI 5.56 to 14.83) were significantly increased by IV iron compared with oral iron. There was a significant reduction in erythropoiesis-stimulating agent (ESA) dose in patients receiving dialysis who were treated with IV iron (9 studies, 487 patients: SMD -0.76, 95% CI -1.22 to -0.30). There was a high level of heterogeneity in all analyses. Mortality and cardiovascular morbidity did not differ significantly, but were reported in few studies. Gastrointestinal side effects were more common with oral iron, but hypotensive and allergic reactions were more common with IV iron. The included studies provide strong evidence for increased ferritin and transferrin saturation levels, together with a small increase in haemoglobin, in patients with CKD who were treated with IV iron compared with oral iron. From a limited body of evidence, we identified a significant reduction in ESA requirements in patients treated with IV iron, and found no significant difference in mortality. Adverse effects were reported in only 50% of included studies. We therefore suggest that further studies that focus on patient-centred outcomes are needed to determine if the use of IV iron is justified on the basis of reductions in ESA dose and cost, improvements in patient quality of life, and with few serious adverse effects.