Rationale: Epidemiological studies suggest that intestinal parasite infections may protect against asthma. Objectives: A systematic review and meta-analysis of epidemiological studies to determine whether total or species-specific current parasite infection is associated with a reduced risk of asthma or wheeze. Methods: We searched MEDLINE, EMBASE and CINAHL (up to January 2006), reviews and reference lists from publications, with no language restrictions. We included studies which reported asthma or wheeze as an outcome measure, and ascertained parasite infection by faecal examination. We estimated pooled odds ratios (OR) and 95% confidence intervals (CI) using data extracted from published papers, or where available, original data provided by authors, using random effect models. Measurements and Main Results: Thirty-three studies met the inclusion criteria. Infection with any parasite was associated with a small, non-significant increase in asthma risk (OR 1.24, 95% CI 0.98 to 1.57, 29 studies). In species-specific analysis, Ascaris lumbricoides was associated with significantly increased odds of asthma (OR 1.34, 95% CI 1.05 to 1.71, 20 studies), whilst hookworm infection was associated with a significantly strong reduction (OR 0.50, 95% CI 0.28 to 0.90; 9 studies) that was directly and significantly related to infection intensity (P<0.001, OR for highest tertile of infection 0.34, 95% CI 0.19 to 0.62). Other species had no significant effects on asthma. Infection effects on wheeze were derived from smaller numbers, but revealed a broadly similar pattern of results. Conclusions: Parasite infections do not in general protect against asthma, but infection with hookworm may reduce the risk of this disease.