If I look at the Boohoo.Com (LSE: BOO) share price chart these days, I can’t help thinking “oh no, it’s happening again.

Since January 2015, Boohoo shares have more than five-bagged to 119p — with an 8% rise today taking the price up 21% since interim results on 27 September. Revenue in the half climbed by 40%, while pre-tax profit soared by 129% with EPS up by 24%, which is very impressive.

So what is it that’s happening again? It’s the ASOS (LSE: ASC) thing. ASOS pioneered pureplay online fashion retailing in the UK, but its investors have had a very rocky ride along the way.

ASOS shares peaked at more than £70 at the beginning of 2014, having five-bagged in just two years, but by August that year the price had fallen two thirds from its zenith and hasn’t since regained that level.

We then saw a climb to more than £40 by April 2015, followed by another slump. And now, just like Boohoo, ASOS shares have stormed ahead again, this time by 186% since October 2014 to £50.70 per share.

Let’s pause for a valuation check on these two…

What’s Boohoo worth?

Analysts are predicting a 50% earnings rise for Boohoo this year, followed by a further 23% in the year to February 2018, which is something that most investors would dream of. But that puts the shares on a very lofty forward P/E of 71, dropping only as far as 58 next year.

To put that into perspective, to get the shares back in line with the FTSE average of around 14, those two forecasts would have to come true and then we’d still need a further quadrupling of earnings!

That would take seven more years of continued earnings growth at the predicted 2018 rate, assuming no further share price rise for the duration — any price appreciation over that time would surely push ‘regular valuation’ day more than a decade away.

What does the PEG say? The PEG, which compares the P/E with the forecast growth rate, tries to quantify a growth share’s potential — with investors typically seeing 0.7 and lower as particularly attractive.

Boohoo shares are on PEG valuations of 1.4 this year and 2.5 next, which don’t look good to me — we really would need that massive decade-long surge in earnings to justify them.

How about ASOS?

At ASOS we see an even scarier picture, with even more elevated P/E multiples of 88 and 69 for the next two years. Analysts suggest earnings growth of around 30% for this year and next, a rate that would need to continue for another six years beyond current forecasts to get that P/E down to average levels (and longer if there’s any share price increase while it happens).

PEG ratios? Even less attractive, at 2.9 and 2.5.

Now, online retail really is a booming business, and fashion is a massive market for it. And while I, in my mature years, prefer to touch and feel the clothes before I consider a purchase, I’m told by those who know that today’s younger shoppers are perfectly happy to order lots of stuff to try in the comfort of their own homes, and send back what they don’t like or what doesn’t fit.

But even with all that going for Boohoo and ASOS, those current valuation figures almost make me want to cry! Couple that with the total absence of dividends, and they’re bargepole shares for me.

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Alan Oscroft has no position in any shares mentioned. The Motley Fool UK owns shares of and has recommended ASOS. The Motley Fool UK has recommended boohoo.com. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors.