The combined oral contraceptive pill is an efficient means of contraception. It acts at different levels of the genital tract. Despite its efficiency, it is universally suggested that patients take the pill at regular daily intervals. Little attention has been given to the question of what happens if you miss the pill one day or more. A study was undertaken to evaluate the consequences of pill misses at different times of the cycle. Forty-seven young, healthy, normally menstruating patients voluntarily enrolled. All were given Cilest (ethinyl estradiol 35 micrograms and norgestimate 250 mg, Cilag France) for 21 days without any misses. Then, after a 7-day interval, they were prescribed one (group 1), two (group 2), three (group 3) or four days of pill misses, to occur respectively on day 1 (group a), 6 (group b), 12 (group c) or 18 (group d) of a new 21 day cycle; supplementary contraceptive means were recommended. Four patients had no miss prescribed and served as controls. Ovarian function was evaluated with daily estrogen measurements (E1 + E2 enzymatic dosage, BioMérieux, France) and ultrasound examinations. When required, because of significant increase in estrogen or because of follicular growth detected on ultrasound, LH and progesterone were measured. None of the patients experienced a normal ovulation. Four patients (1 control, 1 from group 2a, and 2 from group 3a) had a significant increase in estrogen levels and had a follicular image on ultrasounds. One of them (group 3a) had a follicular rupture, but none had a LH surge or increase in progesterone.(ABSTRACT TRUNCATED AT 250 WORDS)