As of the census of 2000, there were 42,389 people, 19,011 households, and 10,779 families residing in the town. The population density was 8,179.6 people per square mile (3,159.6/km2). There were 19,411 housing units at an average density of 3,745.6/sq mi (1,446.8/km2). The racial makeup of the town was 90.97% White, 1.70% African American, 0.13% Native American, 4.97% Asian, 0.01%Pacific Islander, 0.66% from other races, and 1.56% from two or more races. Hispanic or Latino of any race were 1.86% of the population.
There were 19,011 households out of which 23.1% had children under the age of 18 living with them, 45.4% were married couples living together, 8.8% had a female householder with no husband present, 2.2% were unmarried partners, and 43.3% were non-families. 34.2% of all households were made up of individuals and 12.5% had someone living alone who was 65 years of age or older. The average household size was 2.22 and the average family size was 2.91.
In the town the population was spread out with 18.4% under the age of 18, 5.1% from 18 to 24, 36.0% from 25 to 44, 23.7% from 45 to 64, and 16.8% who were 65 years of age or older. The median age was 40 years. For every 100 females there were 86.7 males. For every 100 females age 18 and over, there were 83.1 males.
The median income for a household in the town was $64,344, and the median income for a family was $78,741 (these figures had risen to $77,279 and $98,381 respectively as of a 2007 estimate). Males had a median income of $52,352 versus $40,445 for females. The per capita income for the town was $34,399. About 2.4% of families and 4.1% of the population were below the poverty line, including 2.0% of those under age 18 and 5.5% of those age 65 or over.
Arlington's population in 2000 was a decrease of 20.8% from 53,534 in 1970.