The purpose of this study was to investigate if women with premenstrual dysphoria differ from controls with respect to the number of platelet serotonin transporters, and with respect to three polymorphisms in the gene coding for the serotonin transporter: a 44 base pair insertion/deletion in the promoter region, a variable number of tandem repeats in the second intron, and a single nucleotide polymorphism in the 3' untranslated region. Also, the possible relationship between the three polymorphisms and platelet serotonin transporter density was analyzed. The density of platelet [(3)H]paroxetine binding sites was significantly lower in women with premenstrual dysphoria than in controls, but patients and controls did not differ with respect to allele or genotype frequency for any of the three polymorphisms examined. A significant association between the number of platelet serotonin transporters and the promoter polymorphism was observed, subjects being homozygous for the short (deletion) variant having higher platelet serotonin transporter density than subjects carrying the long (insertion) allele. The results support the assumption that serotonin-related psychiatric disorders-such as premenstrual dysphoria-may be associated with a reduction in platelet [(3)H]paroxetine binding, but argue against the notion that this reduction is due to certain variants of the serotonin transporter gene being more common in patients than in controls.