Accreting millisecond X-ray pulsars had been predicted since early 80's as the progenitors of radio millisecond pulsars. Since then, only a handful have been discovered out of the more than 100 low-mass X-ray binaries, raising yet unresolved questions about the physical difference between pulsating and non pulsating sources. I will report on the recent discovery of two intermittent millisecond X-ray pulsars, discussing possible physical interpretations and their important consequences for our understanding of the X-ray pulsations mechanism.