The destruction of pancreatic β-cells in type 1 diabetes mellitus is mediated by autoreactive T-lymphocyte clones. We initiated a prospective randomized controlled trial of polyclonal rabbit anti-T-cell globulin (ATG) in patients with type 1 diabetes within 4 weeks of diagnosis and with residual post-glucagon C-peptide levels still over 0.3 nmol/l. ATG was administered as an initial bolus of 9 mg/kg followed by 3 consecutive doses of 3 mg/kg. An interim analysis was performed to establish whether any significant changes in C-peptide production and insulin requirement had occurred that would justify the continuation of this pilot study. By May 2004, 11 subjects were assigned to treatment with ATG along with intensified insulin therapy and 6 to intensified insulin therapy with placebo, and were followed for a period of at least 6 months. During the first 12 months a significant difference in the insulin dose trends was found between the groups (p= 0.010) with a lower insulin dosage in the ATG group. There was also a difference in the glucagon stimulated C-peptide level trends of marginal significance (p= 0.068). Compared to values at screening, stimulated C-peptide levels significantly improved in the ATG group (p= 0.012) but not in the placebo group. Complete diabetes remission occurred in 2 patients in the ATG and in none of the placebo group. Glycosylated hemoglobin at 12 months tended to be lower in the ATG group (p= 0.088). Significant adverse effects of ATG treatment, mainly transient fever and moderate symptoms of serum sickness (7 and 6 subjects, respectively) were observed during the first month only. The interim analysis of this ongoing study suggests that short-term ATG therapy in type 1 diabetes of recent onset contributes to the preservation of residual C-peptide production and to lower insulin requirements in the first year following diagnosis.