Serotonin is essential for brain development in early childhood and its production is deficient in autism(1). We previously found low plasma serotonin in mothers of children with autism compared to typical siblings and fathers(2). We report treatment of 8 boys with ASD, 3.5-6 years (mean 4.6) for 4-24 months using citalopram, 0.5-9 mg daily. Children were treated if they had strong family histories of depression; 6 had chronic/recurrent maternal depression and 2 mothers received SSRIs during pregnancy. All had normal genetic and metabolic screening. Citalopram was started at 0.5-1 mg/day and increased weekly by 0.5-1 mg/day as tolerated, avoiding overstimulation as the dose was increased; it was decreased to the previous level if the child showed increased excitation, difficulty with sleep or decreased appetite. Children were treated for up to 1 year and then citalopram was tapered in the same manner as it was started. Within 3 months all 8 patients were much improved on most subscales of the Autism Clinical Global Impression-Improvement (Autism CGI-I) scale. Two patients maintained developmental gains following discontinuation; 2 regressed during weaning and were restarted; 4 are still on initial treatment and doing well. Although citalopram has limited effects in older children with ASD, our experience suggests that it may be effective in promoting brain development in young children, especially in those with a family history of depression and treatment with SSRIs. There are likely to be specific differences in serotonin synthesis or receptors in these families.