The main aim of this retrospective study was to evaluate the occurrence of hypothyroidism among Finnish women with infertility. For this purpose, the records of 335 women presenting for the first time with infertility at the outpatient clinic of reproductive endocrinology at Turku University Central Hospital during a 3-year period (January 1992 to December 1994) were reviewed. Due to missing data, 36 women were excluded from the analysis. Thyroid function was screened by measuring serum thyroid stimulating hormone (TSH) levels in conjunction with serum prolactin using immunoradiometric assays. Prior to enrolment in the infertility examinations, ten out of 299 women had used thyroxine substitution for primary hypothyroidism. In the TSH screening test, 12 women (4%) exhibited elevated serum TSH levels ranging from 5.7 to 32 mU/l. Three of these cases were previously diagnosed with hypothyroidism and were using an inadequate dose of thyroxine. The prevalence of abnormal TSH levels was highest in the ovulatory dysfunction (6.3%) and unknown infertility (4.8%) groups and lowest in the tubal infertility (2.6%) and male infertility (1.5%) groups, although no statistically significant differences between the groups were observed. Oligo/amenorrhea was present in 101 (34%) women in the whole study population and in eight (67%, p < 0.5) women with elevated serum TSH at screening. The relatively high occurrence of abnormal TSH levels in infertile women with ovulatory dysfunctions or unknown infertility, as well as with oligo/amenorrhea, emphasizes the importance of TSH screening in these patient groups.