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Fretting over a possible downturn in Silicon Valley is now a mainstream pursuit.

Marc Andreessen, the prominent venture capitalist, took to Twitter on Thursday to warn against excessive spending by start-ups that have attracted capital from investors. Companies that spend money on fancy offices or too many employees, he said, could be in trouble when the market turns.

Mr. Andreessen is one of several technology insiders to recently raise such concerns. DealBook reported in August that, with capital flowing freely and start-up valuations soaring, some start-ups were raising cash as an insurance policy against leaner times. Bill Gurley, a partner at the venture capital firm Benchmark, warned in an interview with The Wall Street Journal that “no one’s fearful, everyone’s greedy, and it will eventually end.” Fred Wilson, a partner at Union Square Ventures, later wrote a blog post about excessive “burn rates.”

But Mr. Andreessen’s Twitter lecture was notable because he has been one of the most vocal opponents of the idea that Silicon Valley is currently in a bubblelike environment.