SAN FRANCISCO — Tens of millions of baby boomers are encountering the signs, by turns amusing and disconcerting, that accompany the decline of the brain's acuity: a good friend's name suddenly vanishing from memory; a frantic search for eyeglasses only to find them atop the head; milk taken from the refrigerator then put away in a cupboard.

At the same time, boomers are seizing on a mounting body of evidence that suggests that brains contain more plasticity than previously thought, and many people are taking matters into their own hands, doing brain fitness exercises with the same intensity with which they attack a treadmill.

Decaying brains, or the fear thereof, have inspired a mini-industry of brain health products — not just supplements like coenzyme Q10, ginseng and bacopa, but computer-based fitter-brain products as well. Nintendo's $19.99 Brain Age 2, a popular video game of simple math and memory exercises, is one. Posit Science's $395 computer-based "cognitive behavioral training" exercises are another. MindFit, a $149 software-based program, combines cognitive assessment of more than a dozen skills with a personalized training regimen based on that assessment. And for about $10 a month, worried boomers can subscribe to Web sites like Lumosity.com and Happy-Neuron.com, which offer a variety of cognitive training exercises.

Alvaro Fernandez, whose brain fitness and consulting company, SharpBrains, has a Web site focused on brain fitness research, estimates that in 2007 the market in the United States for so-called neurosoftware was $225 million.

Fernandez pointed out that compared with, say, the physical fitness industry, which brings in $16 billion a year in health club memberships alone, the brain fitness software industry is still in its infancy. Yet it is growing at a 50 percent annual rate, he said, and he expects it to reach $2 billion by 2015.

From Hula Hoops to Corian countertops, marketers have done very well over the six decades guessing the desires of the generation born after World War II. Now they are making money on that generation's fears, and it is not just computerized flash card makers with the money-making ideas. Doctors and geneticists have also tapped into the market.

Boomers believe they have ample reason to worry. There is no definitive laboratory test to detect Alzheimer's disease. Doctors rely on symptoms to make the diagnosis, and most think that by the time symptoms show up the brain damage is already extensive.

By 2050, according to the Alzheimer's Association, 11 million to 16 million Americans will have the disease.