To determine whether there is a specific temporal risk for opioid drug overdose. DESIGN: To study patients presenting to the ED in a comatose state from accidental drug opioid overdose. PARTICIPANTS: Two hundred seventy-four patients were admitted to the ED of the Hospital of Ferrara, Italy, from 1988 to 1990, 225 men (82.1%; mean age, 25 +/- 3.4 years) and 49 women (17.9%; mean age, 23.5 +/- 2.8 years). INTERVENTIONS: Month, day, and hour and minute of admissions were recorded, and time-qualified frequency data were analyzed by the single cosinor method. RESULTS: Cosinor analysis demonstrated a significant circadian rhythm for both the total number of observations and the separate male and female subgroups with an early evening peak ("acrophase") at about 7:00 PM. No significant circannual rhythm was evident, but for the total group a significant 6-month rhythm was demonstrable with peaks in late November and late May. CONCLUSION: There is a distinct "chronorisk" of opioid drug overdose in the early evening hours.