Exposure to ambient nitrogen dioxide (NO2) has been linked to increased mortality in several epidemiological studies but the question remains of whether NO2 is directly responsible for the health effects or is only an indicator of other pollutants, including particulate matter. The aim of the present review was to provide pooled estimates of the long-term effects of NO2 on mortality, which are potentially useful for health impact assessment. We selected 23 papers, published from 2004 to 2013, evaluating the relationship between NO2 and mortality, also including an assessment of the effect of particulate matter exposure. A random-effects meta analysis was carried out on 19 studies. The pooled effect on mortality was 1.04 (95% CI 1.02–1.06) with an increase of 10 mg?m-3 in the annual NO2 concentration and 1.05 (95% CI 1.01–1.09) for particulate matter ,2.5 mm in diameter (PM2.5) (10 mg?m-3). The effect on cardiovascular mortality was 1.13 (95% CI 1.09–1.18) for NO2 and 1.20 (95% CI 1.09–1.31) for PM2.5. The NO2 effect on respiratory mortality was 1.03 (95% CI 1.02–1.03) and 1.05 (95% CI 1.01–1.09) for PM2.5. Four bipollutant analyses with particulate matter and NO2 in the samemodels showed minimal changes in the effect estimates of NO2. There is evidence of a long-term effect of NO2 on mortality as great as that of PM2.5. An independent effect of NO2 emerged from multipollutant models.