Background: Although physical fitness has been suggested to improve physical and psychosocial health for a variety of population profiles, there is a lack of information about the safety and effectiveness of aerobic exercise for adults with Down syndrome. Objectives: To evaluate the effectiveness and safety of aerobic exercise training programmes for physiological and psychosocial outcomes in adults with Down syndrome. Search strategy: Search terms and synonyms for "aerobic exercise" and "Down syndrome" were used within the following databases: CENTRAL (2007, Issue 1); MEDLINE via PUBMED (1966 to March 2007); EMBASE (2005 to April 2007); CINAHL (1982 to March 2007); LILACS (1982 to March 2007); PsycINFO (1887 to March 2007); ERIC (1966 to March 2007); CCT (March 2007); Academic Search Elite (to March 2007), C2- SPECTR (to March 2007 ), NRR (2007 Issue 1), ClinicalTrials.gov (accessed March 2007) and within supplements of Medicine and Science in Sports and Exercise. Selection criteria: Randomised or quasi-randomised controlled trials using supervised aerobic exercise training programmes with behavioral components accepted as co-interventions. Data collection and analysis: Two reviewers selected relevant trials, assessed methodological quality and extracted data. Where appropriate, data was pooled using meta-analysis with a random effects model Main results: The two studies included in this trial used different kinds of aerobic activity: walking/jogging and rowing training. One included study was conducted in the USA, the other in Portugal. In the meta-analyses, only maximal treadmill grade, a work performance variable, was improved in the intervention group after aerobic exercise training programmes (-4.26 [95% CI -6.45, -2.06]) grade. The other outcomes in the meta-analysis showed no significant differences between intervention and control groups, as expressed by weighted mean difference: VO(2) peak -0.30 (95% CI -377, 3.17) mL.Kg.min(-1); peak heart rate, -2.84 (95% CI -10.73, 5.05) bpm; respiratory exchange ratio, 0.01 (95% CI -0.04, 0.06); pulmonary ventilation, -5.86 (95% CI -16.06, 4.34) L.min(-1). 30 other measures including work performance, oxidative stress and body composition variables could not be combined in the meta-analysis. Apart from work performance, trials reported no significant improvements in these measures. Authors' conclusions: There is insufficient evidence to support improvement in physical or psychosocial outcomes of aerobic exercise in adults with Down syndrome. Although evidence exists which supports improvements in physiological and psychological aspects from strategies using mixed physical activity programmes, well-conducted research which examines long-term physical outcomes, adverse effects, psychosocial outcomes and costs are required before informed practice decisions can be made.