As of the census of 2000, there were 502,266 people, 167,542 households, and 131,017 families residing in the county. The population density was 600 people per square mile (232/km²). There were 175,524 housing units at an average density of 210 per square mile (81/km²). The racial makeup of the county was 81.83% White, 10.45% Black or African American, 0.21% Native American, 2.21% Asian, 0.03% Pacific Islander, 3.63% from other races, and 1.63% from two or more races. 8.71% of the population were Hispanic or Latino of any race. 18.1% were of German, 12.8% Irish, 10.1% Polish and 9.8% Italian ancestry according to Census 2000.
There were 167,542 households out of which 42.70% had children under the age of 18 living with them, 64.80% were married couples living together, 9.60% had a female householder with no husband present, and 21.80% were non-families. 17.80% of all households were made up of individuals and 6.00% had someone living alone who was 65 years of age or older. The average household size was 2.94 and the average family size was 3.36.
In the county the population was spread out with 30.00% under the age of 18, 8.10% from 18 to 24, 32.90% from 25 to 44, 20.60% from 45 to 64, and 8.30% who were 65 years of age or older. The median age was 33 years. For every 100 females there were 99.80 males. For every 100 females age 18 and over, there were 97.40 males.
The median income for a household in the county was $62,238, and the median income for a family was $69,608 (these figures had risen to $73,159 and $82,082 respectively as of a 2007 estimate). Males had a median income of $50,152 versus $31,345 for females. The per capita income for the county was $24,613. About 3.40% of families and 4.90% of the population were below the poverty line, including 5.60% of those under age 18 and 5.50% of those age 65 or over.