['To investigate whether variable antidepressant response may be influenced by an interaction between the serotonin transporter promoter polymorphism (5-HTTLPR) and antidepressant concentration.', 'Elderly subjects with depression treated with paroxetine (n = 110) were genotyped and assessed with the Hamilton Rating Scale for Depression (HAMD). A mixed-effect analysis of repeated measures was used.', 'There was an interaction between early paroxetine concentration and 5-HTTLPR genotype on symptomatic improvement over 12 weeks (F(18,59.5) = 1.8, p < 0.05), as well as main effects of both paroxetine concentration (F(68,55.3) = 2.4, p < 0.005) and genotype (F(2,74.2) = 5.7, p < 0.005). Paroxetine concentrations were correlated with change in HAMD scores after 2 weeks of treatment in subjects with the short (s) allele (r = 0.31, p < 0.05) but not in subjects homozygous for the long (l) allele.'] Does serotonin transporter genotype interact with paroxetine plasma levels to influence depression treatment response in geriatric patients?