BACKGROUND: Lay health workers (LHWs) are widely used to provide care for a broad range of health issues. However, little is known about the effectiveness of LHW interventions. OBJECTIVES: To assess the effects of LHW interventions in primary and community health care on health care behaviours, patients' health and wellbeing, and patients' satisfaction with care. SEARCH STRATEGY: We searched the Cochrane Effective Practice and Organisation of Care and Consumers and Communication specialised registers (to August 2001); the Cochrane Central Register of Controlled Trials (to August 2001); MEDLINE (1966- August 2001); EMBASE (1966-August 2001); Science Citations (to August 2001); CINAHL (1966-June 2001); Healthstar (1975-2000); AMED (1966-August 2001); the Leeds Health Education Effectiveness Database and the reference lists of articles. SELECTION CRITERIA: Randomised controlled trials of any intervention delivered by LHWs (paid or voluntary) in primary or community health care and intended to promote health, manage illness or provide support to patients. A 'lay health worker' was defined as any health worker carrying out functions related to health care delivery; trained in some way in the context of the intervention; and having no formal professional or paraprofessional certificated or degreed tertiary education. There were no restrictions on the types of consumers. DATA COLLECTION AND ANALYSIS: Two reviewers independently extracted data onto a standard form and assessed study quality. Studies that compared broadly similar types of interventions were grouped together. Where feasible, the results of included studies were combined and an estimate of effect obtained. MAIN RESULTS: Forty three studies met the inclusion criteria, involving more than 210,110 consumers. These showed considerable diversity in the targeted health issue and the aims, content and outcomes of interventions. Most were conducted in high income countries (n=35), but nearly half of these focused on low income and minority populations (n=15). Study diversity limited meta-analysis to outcomes for five subgroups (n=15 studies) (LHW interventions to promote the uptake of breast cancer screening, immunisation and breastfeeding promotion [before two weeks and between two weeks and six months post partum] and to improve diagnosis and treatment for selected infectious diseases). Promising benefits in comparison with usual care were shown for LHW interventions to promote immunisation uptake in children and adults (RR=1.30 [95% CI 1.14, 1.48] p=0.0001) and LHW interventions to improve outcomes for selected infectious diseases (RR=0.74 [95% CI 0.58, 0.93) p=0.01). LHWs also appear promising for breastfeeding promotion. They appear to have a small effect in promoting breast cancer screening uptake when compared with usual care. For the remaining subgroups (n=29 studies), the outcomes were too diverse to allow statistical pooling. We can therefore draw no general conclusions on the effectiveness of these subgroups of interventions. AUTHORS' CONCLUSIONS: LHWs show promising benefits in promoting immunisation uptake and improving outcomes for acute respiratory infections and malaria, when compared to usual care. For other health issues, evidence is insufficient to justify recommendations for policy and practice. There is also insufficient evidence to assess which LHW training or intervention strategies are likely to be most effective. Further research is needed in these areas.