Nine hospital workload factors and seasonal changes in daylight and darkness were examined over a 5-year period in relation to nurse medication errors at a medical center in Anchorage, Alaska. Three workload factors, along with darkness, were found to be significant predictors of the risk of medication error. Errors increased with the number of patient days per month (OR/250 patient days = 1.61) and the number of shifts worked by temporary nursing staff (OR/10 shifts = 1.15); errors decreased with more overtime worked by permanent nursing staff members (OR/10 shifts = .85). Medication errors were 95% more likely in midwinter than in the fall, but the effect of increasing darkness was strongest; a 2-month delay was found between the level of darkness and the rate of errors. More than half of all medication errors occurred during the first 3 months of the year.