Nigeria is the most populous country in Africa but exactly how populous is a subject of speculation. The United Nations estimates that the population in 2009 was at 154,729,000, distributed as 51.7% rural and 48.3% urban, and with a population density of 167.5 people per square kilometer. National census results in the past few decades have been disputed. The results of the most recent census were released in December 2006 and gave a population of 140,003,542. The only breakdown available was by gender: males numbered 71,709,859, females numbered 68,293,08.

According to the United Nations, Nigeria has been undergoing explosive population growth and one of the highest growth and fertility rates in the world. By their projections, Nigeria will be one of the countries in the world that will account for most of the world's total population increase by 2050.

According to current data, one out of every four Africans is Nigerian. Presently, Nigeria is the eighth most populous country in the world, and even conservative estimates conclude that more than 20% of the world's black population lives in Nigeria. 2006 estimates claim 42.3% of the population is between 0–14 years of age, while 54.6% is between 15–65; the birth rate is significantly higher than the death rate, at 40.4 and 16.9 per 1000 people respectively.

Health, health care, and general living conditions in Nigeria are poor. Life expectancy is 47 years (average male/female) and just over half the population has access to potable water and appropriate sanitation; the percentage is of children under five has gone up rather than down between 1990 and 2003 and infant mortality is 97.1 deaths per 1000 live births. HIV/AIDS rate in Nigeria is much lower compared to the other African nations such as Kenya or South Africa whose prevalence (percentage) rates are in the double digits. In 2003, the HIV prevalence rate among 20 to 29 year-olds was 5.6%. About Nigeria, like many developing countries, suffers from a polio crisis as well as periodic outbreaks of cholera, malaria, and sleeping sickness. As of 2004, there has been a vaccination drive, spearheaded by the W.H.O., to combat polio and malaria that has been met with controversy in some regions.

Education is also in a state of neglect. After the 1970s oil boom, tertiary education was improved so that it would reach every subregion of Nigeria. Education is provided free by the government, but the attendance rate for secondary education is only 29% (32% for males, 27% for females). The education system has been described as "dysfunctional" largely because of decaying institutional infrastructure. 68% of the population is literate, and the rate for men (75.7%) is higher than that for women (60.6%).

Nigeria's largest city is Lagos. Lagos has grown from 300,000 in 1950 to an estimated 15 million today, and the Nigerian government estimates that city will have expanded to 25 million residents by 2015.

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