Objectives We aim to explore undertriage and overtriage in a high-risk patient population and explore patient characteristics and call characteristics associated with undertriage and overtriage in both randomly selected and in high-risk telephone calls to out-of-hours primary care (OOH-PC). Design Natural quasi-experimental cross-sectional study. Setting Two Danish OOH-PC services using different telephone triage models: a general practitioner cooperative with GP-led triage and the medical helpline 1813 with computerised decision support system-guided nurse-led triage. Participants We included audio-recorded telephone triage calls from 2016: 806 random calls and 405 high-risk calls (defined as patients ≥30 years calling with abdominal pain). Main outcome measures Twenty-four experienced physicians used a validated assessment tool to assess the accuracy of triage. We calculated the relative risk (RR) for clinically relevant undertriage and overtriage for a range of patient characteristics and call characteristics. Results We included 806 randomly selected calls (44 clinically relevant undertriaged and 54 clinically relevant overtriaged) and 405 high-risk calls (32 undertriaged and 24 overtriaged). In high-risk calls, nurse-led triage was associated with significantly less undertriage (RR: 0.47, 95% CI 0.23 to 0.97) and more overtriage (RR: 3.93, 95% CI 1.50 to 10.33) compared with GP-led triage. In high-risk calls, the risk of undertriage was significantly higher for calls during nighttime (RR: 2.1, 95% CI 1.05 to 4.07). Undertriage tended to be more likely for calls concerning patients ≥60 years compared with 30-59 years (11.3% vs 6.3%) in high-risk calls. However, this result was not significant. Conclusion Nurse-led triage was associated with less undertriage and more overtriage compared with GP-led triage in high-risk calls. This study may suggest that to minimise undertriage, the triage professionals should pay extra attention when a call occurs during nighttime or concerns elderly. However, this needs confirmation in future studies.