According to the 2000 Census there were 38,000 people, 13,000 households, and 9,900 families living in Crystal Lake. The population density was 900/km² (2,300/sq mi). There were 13,500 housing units at an average density of 830/sq mi (320/km²). The racial makeup of the city was 94% white, 0.56% black, 0.16% Native American, 2% Asian, 0.02% Pacific Islander, 2% from other races, and 1% from two or more races. Hispanic or Latino of any race were 7% of the population.
45% of the households included children under 18, 64% were married couples living together, 8% had a female householder with no husband present, and 25% were non-families. 20% of all households were made up of individuals and 8% had someone at least 65 years old living alone. The average household size was 2.9 and the average family size was 3.4.
In the city the population was spread out with 32% under the age of 18, 7% from 18 to 24, 33% from 25 to 44, 20% from 45 to 64, and 9% who were 65 years of age or older. The median age was 34 years. For every 100 females there were 98 males. For every 100 females age 18 and over, there were 93 males.
The median income for a household in the city was $67,000, and the median income for a family was $75,000 (these figures had risen to $69,816 and $84,935 respectively as of a 2007 estimate). Males had a median income of $52,000 versus $32,000 for females. The per capita income for the city was $26,000. About 3% of families and 4% of the population were below the poverty line, including 3% of those under age 18 and 4% of those age 65 or over.