Great news for students - jeans left unwashed for a year are no dirtier than those that have been worn for a fortnight.
A New Zealand scientist based in Canada used a live subject - an Alberta student who refused to clean his jeans - to test how dirty they can get.
But surprising results showed that the denim carried no more bacteria after 15 months of not washing than after two weeks.
University of Alberta student Josh Le, 20, did not wash his jeans in the name of fashion - the "raw" denim look, in which wearers seek a creased and faded style.
New Zealand researcher Rachel McQueen, his human ecology lecturer and an expert in fabric and odour, saw the opportunity for a small, experimental study.
Dr McQueen, a University of Otago graduate now working in Edmonton, measured the amount of filth on his jeans and compared the results to the same jeans after being washed and worn for a fortnight.
"I thought that the bacterial counts would be relatively high [after 15 months]. But I was quite surprised. After a shorter time of wearing them the bacterial counts were roughly about the same."
"I thought I'd find some E.Coli ... and there wasn't. [The bacteria] is not at any level which would cause concern."
The crotch region of the jeans had the highest bacteria count - between 8,500 and 10,000 bacterial units per square centimetres - but none of this bacteria was harmful.
She said the results were highly dependent on the individual, and this study would not upset washing machine or detergent makers.
Her subject was vigilant about hygiene, wore underwear, was fit and healthy, and also looked after his jeans by airing them regularly.
"His jeans are unlikely to be as manky as someone who wasn't as hygienic as he was. You could smell the jeans from about 2cm away but other than that they weren't too bad."
Dr McQueen said that stringent laundering treatments were important in many workplaces, such as hospitals, kitchens and hotels.
But for most everyday clothing the effect of washing some items less frequently had greater potential benefits to the environment than potential risk to the wearer.
She referred to a study by jeans maker Levi-Strauss, which researched the life cycle of a pair of jeans.
The Levi-Strauss investigation concluded that by washing jeans once a month, instead of once a week, wearers could reduce their energy use by about 40 per cent and water use by about 35 per cent.
HOW OFTEN ARE JEANS WASHED?
Sebastian Suckling, 19
Never. I would only ever wash them if I spilt something on them. Partly because my washing machine is really bad. The longest a pair has lasted is three years, then they just fell apart.
Amy Brent-Jones, 16
Every two days. I don't use a dryer every time, I just hang them out on the line. So I guess it's not too bad on the environment.
Shivnil Gonnder, 19
I would wash them once a week. I get quite sweaty. I'm still a student, so my mum washes them - that's why they're done so often.