PIP: A double-blind crossover study of 480 women taking either Ovral, Norinyl, or Norlestrin was conducted to determine the effects of missing 1 contraceptive tablet in the pill sequence on the incidence of breakthrough bleeding. The women were instructed to take 2 tablets the following day in the case of a missed tablet. The probability of breakthrough bleeding among those women who did not miss a tablet was in the range of .017 to .022, but those who had missed a tablet had a probability .227 to .352. However, the duration of breakthrough bleeding was somewhat less for those who missed a tablet than for those who maintained a regular schedule. The range in probability reflects differences between the compounds used. The results strongly suggest that the risk of breakthrough bleeding is considerably increased when an oral contraceptive pill is omitted from the pill cycle.