TOKYO -- The almanacs may not mention it, but one of last summer's more momentous events in Japan was the first sighting of Prime Minister Junichiro Koizumi in an open-necked white shirt. Without a necktie. Addressing parliamentary committees!

In a nation noted for both its buttoned-up, dark-suited work force and sticky summers, the message was clear: Be cool. In fact, Mr. Koizumi's appearance marked the inception of Cool Biz, a campaign run by the government's environment ministry to encourage men to leave their ties and jackets at home during the sweltering summer months -- the idea being to cut down on air-conditioning use.

Now, another summer is in the offing, and Cool Biz is back -- with good reason. According to the government, by urging corporate Japan to set office temperatures at a steamy 28° Celsius (82° Fahrenheit), the nation in 2005 reduced carbon-dioxide emissions by 460,000 tons -- the same amount that a million households create each month.

This summer, Japan's fashion houses are rolling out a dizzying array of form-flattering shirts, jackets and trousers in ultra-lightweight fabrics to keep "salary men" from sweating too much during the dog days. Taking a cue from Prime Minister Koizumi's wardrobe last summer -- which grew to include shirts in colors like mauve and yellow -- shirt makers have added a rainbow of colors, from cranberry reds to minty greens, to their palettes. Department stores are pushing pink shirts, but Akemi Sugawara, president of fashion consultancy Impression Inc. and a clothes adviser to prominent Japanese businessmen, thinks that goes too far. Her verdict: "Too sweet and childish."The rush of new styles puts Japanese men at a historic fashion crossroads: Should they take the low road, and dress down for comfort? Or stick to the high road, and remain buttoned down for appearance?