To investigate the risk of frost damage to Scots pine (Pinus sylvestris L.) in northern regions under climatic warming, a submodel for such damage to trees was included in a forest ecosystem model of the gap type. An annual growth multiplier describing the effects of frost was calculated with the help of simulated daily frost hardiness and daily minimum temperature. The annual growth multiplier was used in the main ecosystem model when simulating the development of a tree stand using a time step of one year. Simulations of the growth and development of Scots pine stands in southern Finland (60°N) under an elevating temperature indicated that climatic warming could increase the risk of frost damage due to premature onset of growth during warm spells in the late winter and early spring. Risk of frost damage implies uncertainty in yield expectations from boreal forest ecosystems in the event of climatic warming.