OBJECTIVES: The  changes  of  vitamin  D  level  influencing  bone metabolism  are  seasonal,  the  effect  of  seasons  on  femoral neck  fracture  healing  is  unknown. The aim of our study was to determine the effects of seasonal periodicity on incidence of osteonecrosis following primary osteosynthesis of femoral neck fractures in Hungarian elderly population. METHODS: This nationwide retrospective observational cohort study based on dataset of National Health Insurance Fund Administration. Patients over 60 years undergoing internal fixation after primary femoral neck fracture (ICD10-S7200) and were discharged in 2000 were identified. Inpatient cases developing osteonecrosis were registered during the 8 years follow-up. The following factors were evaluated: sex, age, type of fracture, season of surgical intervention, day of surgery, surgical delay and comorbidities. The effect of seasons on survival time was investigated by Kaplan–Meier analysis and log-rank test. The crude incidence rate ratios of osteonecrosis were calculated for factors. The impact of risk factors on the hazard of osteonecrosis was estimated by Cox proportional hazard model. RESULTS: CONCLUSIONS: Wintertime osteosynthesis increases the risk of osteonecrosis. The findings raise  the  possibility  of  association  between  seasonal changes in vitamin D levels and impaired fracture healing of femoral neck fracture. The results may help establish an effective strategy for the prevention of serious complications.