Serum levels of thyroxine, triiodothyronine, triiodothyronine resin uptake, thyroxine-binding globulin, thyrotropin, and ultrasonically determined thyroid gland volume were investigated weekly in 11 healthy women during a normal menstrual cycle, and in five healthy men once a week for 5 consecutive weeks. In the men the thyroid volume was unaltered. In the women, however, a mean variation in thyroid volume of approximately 50% between minimum values (15.4 +/- 3.1 ml, day 9) and maximum values (24.4 +/- 4.8 ml, day 23) was found (p less than 0.01), although no significant differences in the other thyroid variables could be demonstrated. The menstrual cycle seems to be associated with cyclic alterations of thyroid size in healthy women unexplained by alterations in thyroid function variables. This information should be taken into account when goiter frequency, goitrogenic action of drugs, and goiter treatment effects are evaluated.