Pre-clinical and few clinical studies suggest that transplantation of autologous bone marrow mononuclear cells (BMNC) improves heart function in dilated cardiomyopathies. Our objective was to determine if intracoronary injection of autologous BMNC improves the left ventricular ejection fraction (LVEF) of patients with non-ischaemic dilated cardiomyopathy (NIDCM). This study was a multicentre, randomized, double-blind, placebo controlled trial with a follow-up of 12 months. Patients with NIDCM and LVEF <35% were recruited at heart failure ambulatories in specialized hospitals around Brazil. One hundred and sixty subjects were randomized to intracoronary injection of BMNC or placebo (1:1). The primary endpoint was the difference in change of LVEF between BMNC and placebo groups as determined by echocardiography. One hundred and fifteen patients completed the study. Left ventricular ejection fraction decreased from 24.0% (21.6-26.3) to 19.9% (15.4-24.4) in the BMNC group and from 24.3% (22.1-26.5) to 22.1% (17.4-26.8) in the placebo group. There were no significant differences in changes between cell and placebo groups for left ventricular systolic and diastolic volumes and ejection fraction. Mortality rate was 20.37% in placebo and 21.31% in BMNC. Intracoronary injection of autologous BMNC does not improve left ventricular function in patients with NIDCM. URL: http://www.clinicaltrials.gov. Unique identifier: NCT00333827.