(Fortune Magazine) -- During the 16 times that Fortune has compiled our list of the 100 Fastest-Growing Companies, we've learned a few things. One is that sustaining growth at these levels ain't easy. Only 43 of the 100 companies from last year's list made the grade again this year. Another is that being in the right place at the right time can be just as important as how you run your business. The latest example: In 2005, oil prices averaged $52 a barrel, up from $38 a barrel in 2004. Voilà! Almost a third of the companies on this year's list are in the energy sector, including No. 1, Vaalco Energy.

To compute the rankings, we use data from Zacks Investment Research and give equal weight to three factors: profit and sales growth (for three years through the first quarter of 2006) and three-year total return (through June). For additional details, see fortune.com.

By L. Michael Cacace (Senior List Editor), Douglas Elam, Eugenia Levenson, Jenny Mero, Anastasia Serdyukova, Christopher Tkaczyk and Richard K. Tucksmith