Monthly binding densities (B(max)) of [3H]citalopram to the platelet serotonin transporter (SERT) was measured longitudinally over 1 year in a control group of 18 healthy Finnish male volunteers. Single platelet samples were also analysed from 33 men who were incarcerated for violent crimes during the same calendar year. A statistically significant seasonal variation in SERT B(max) was observed in both data sets, and bi-monthly floating averages for SERT B(max) were calculated and then fit to an annual sinusoidal curve for both groups. The B(max) for platelet [3H]citalopram binding showed a statistically significant (p = 0.001) seasonal variance between a winter (January-February) maximum of 1590 fmol/mg protein and a summer (July-August) minimum of 1216 fmol/mg protein for the control group, with an R2 of 70% for the annual sinusoidal curve fit. A statistically significant (p = 0.007) seasonal variance was also observed between a winter (January-February) maximum of 1980 fmol/mg protein and an autumnal (August-September) minimum of 1234 fmol/mg protein for the violent offenders, again with an R2 of 70% for the annual sinusoidal curve fit. This observation lends additional support to the idea that violent human behavior and impulsivity may be directly linked to values of SERT B(max), which can be affected by various psychoactive drugs and also varies with the natural change of seasons.