Background: Annual patterns in suicide rates, peaking near the summer solstice, are well documented. It has been suggested that day length or total hours of sunshine has an impact on suicide rates. If these environmental factors are involved, we would expect changes in the daily pattern of suicide rates to occur over the year. To test this hypothesis, the 24-h patterns of suicide rate were investigated as a function of time of year. Method: Detailed information about the exact time of suicides in The Netherlands is only available for train suicides. Therefore, information concerning age, sex, time and place of occurrence of all verified train suicides over 15 years in The Netherlands (n = 2830) was obtained from The Netherlands Railways archives. Results: Daily patterns in train suicides show systematic variations of two kinds. First, independently of time of year, suicide rates at night drop to about 10% of their daytime values. Second. there are two daily peaks in the patterns which shift their timing over the year, with one peak occurring shortly after sunset, and the other one consistently occurring 9-10 h earlier. Both peaks shift with the 5.5-h shift in sunset time. Limitations: Train suicidal behaviour may not represent fatal suicidal behaviour in general. Conclusions: There are pronounced and systematic daily variations in train suicide rates in The Netherlands. One of these is related to clock time, the others are related to the light-dark cycle. The consistency of the patterns suggests a strong environmental influence on train suicidal behaviour. Research on 24-h patterns of suicide rates should control for time of year. (C) 2001 Elsevier Science B.V. All rights reserved.