Featuring spectacular views of the San Gabriel Mountains, Altadena was developed as a suburb of Pasadena in 1887. It has long refused annexation by neighboring Pasadena. Its unincorporated status allowed it to become a major settlement area for working- and middle-class African-Americans during a period in which non-whites were forbidden to live in the vast majority of greater Los Angeles.
The name Altadena derives from the Spanish alta, meaning "upper", and -dena from Pasadena; the area is adjacent to, but at a higher elevation than, Pasadena.
As of the census of 2000, there were 42,610 people, 14,780 households, and 10,671 families residing in the CDP. The population density was 4,898.9 people per square mile (1,891.0/km²). There were 15,250 housing units at an average density of 1,753.3/sq mi (676.8/km²). The racial makeup of the CDP was 47.30% White, 31.42% Black or African American, 0.58% Native American, 4.24% Asian, 0.13% Pacific Islander, 10.19% from other races, and 6.14% from two or more races. 20.39% of the population were Hispanic or Latino of any race.
There were 14,780 households out of which 34.1% had children under the age of 18 living with them, 52.3% were married couples living together, 15.2% had a female householder with no husband present, and 27.8% were non-families. 21.8% of all households were made up of individuals and 7.1% had someone living alone who was 65 years of age or older. The average household size was 2.82 and the average family size was 3.29.
In the CDP the population was spread out with 26.6% under the age of 18, 6.4% from 18 to 24, 30.1% from 25 to 44, 24.6% from 45 to 64, and 12.3% who were 65 years of age or older. The median age was 38 years. For every 100 females there were 91.8 males. For every 100 females age 18 and over, there were 86.6 males.
The median income for a household in the CDP was $60,549, and the median income for a family was $66,800 (these figures had risen to $77,020 and $86,778 respectively as of a 2007 estimate). Males had a median income of $49,098 versus $38,054 for females. The per capita income for the CDP was $27,604. About 7.4% of families and 10.6% of the population were below the poverty line, including 14.1% of those under age 18 and 8.3% of those age 65 or over.