BACKGROUND Diabetes mellitus type 1 is a chronic disease with short and long term complications. Its goals of therapy are to eliminate the symptoms of hyperglycaemia, reduce the long term microvascular and macrovascular complications and allow the patients to achieve a normal life-style. Basal insulin replacement for insulin dependent patients can be achieved with either intermediate or long acting insulin preparations. OBJECTIVE To assess the effects of intermediate acting versus long acting insulin preparations for basal insulin replacement in type 1 diabetic patients. METHODS We searched MEDLINE, EMBASE and The Cochrane Library, as well as reference lists, databases of ongoing trials, and requests from authors of included trials. METHODS Randomised controlled trials, assessing long acting insulin preparations compared to intermediate acting insulin preparations, in type 1 diabetic patients. METHODS Two reviewers independently scanned the titles. Data were extracted and analysed accordingly. RESULTS Twenty-three randomised controlled trials were identified. A total of 3872 and 2915 participants in the intervention and in the control group, respectively, were analysed. The weighted mean difference (WMD) for the level of glycosylated haemoglobin was -0.08 (95% confidence interval (CI) -0.12 to -0.04) in favour of the long acting insulin arm. The WMD between the groups in fasting plasma and blood glucose levels was -0.63 (95% CI -0.86 to -0.40) and -0.86 (95% CI -1.00 to -0.72) in favour of the long acting insulins. The odds ratio for a patient on long acting insulin to develop any type of hypoglycaemia was 0.93 (95% CI 0.8 to 1.08) compared to that of a patient on intermediate acting insulins. The OR for severe hypoglycaemic episodes was 0.73 (95% CI 0.61 to 0.87), and 0.70 (95% CI of 0.63 to 0.79) for nocturnal episodes. The WMD between the long and intermediate insulin groups for hypoglycaemic events per 100 patient follow up days was -0.77 (95% CI -0.89 to -0.65), -0.0 (95% CI -0.02 to 0.02) and -0.40 (95% CI -0.45 to -0.34) for overall, severe, and nocturnal hypoglycaemic episodes. Weight gain was more prominent in the control group. No difference was noted in the quantity or quality of severe adverse events or deaths. CONCLUSIONS Long acting insulin preparations seem to exert a beneficial effect on nocturnal glucose levels. Their effect on the overall diabetes control is clinically unremarkable. Their use as a basal insulin regimen for type 1 diabetes mellitus warrants further substantiation.