Nearly 10 million households in the US have a net worth of at least $1 million, excluding their primary residences, according to TNS's "Affluent Market Research Program."

The research company said the number of millionaire households increased by 5.9% from June 2006 to June 2007. It was the sixth such increase in as many years.

The average age in millionaire households was 66, with an average net worth of $4.6 million. The single most important financial goal of surveyed millionaires (56%) was to "assure a comfortable standard of living during retirement."

Nearly six out of 10 millionaires surveyed said their investment approach had changed very little since the previous year. More than one-third said they "took a wait-and-see approach" toward investing and about one-quarter looked for investment bargains.