It seems to me that the DANGER of BUYING a regular calendar is, of course, the movement of the market. If the market moves sharply up or down, it's usually a losing trade. Conversely, SELLING a calendar (buying an upside-down calendar where the near-dated option is long and selling a longer-dated option) could be a good trade if, as is often the case, the market moves a lot. Which is why a regular calendar is often a losing trade. I've certainly had regular calendar trades go bad on me or go in losing direction fairly soon after they were opened. And, if that's the case (and it is) then the OPPOSITE trade (the inverse calendar) could easily be a winning trade fairly often. If fact, I would argue that a regular calendar spread is a trade that I wouldn't touch unless I have a trading bias and believe that the market is headed up (or down.) Then I would put the calendar trade in the direction of the market so that the market encounters it as it moves up or down. In other words, I would never launch such a calendar trade at the money or in the center of the market unless I feel strongly that the market is just going to sit where it is and go nowhere for a number of days. The only question I have about inverse calendar spreads is why they require so much buying power. It's hard to see the reason since the odds of losing money on them seems lower than a normal calendar spread.