For a considerable coverage the energy balance of and ice formation by leads in sea ice in the Weddell Sea are evaluated on the basis of data obtained from drifting buoys for the winter periods from 1986 to 1994 and by using a kinematic-thermodynamic sea ice model. The net heat flux is defined as the sum total of radiative and turbulent fluxes. For thin ice the net turbulent flux is 3--4 times the net radiative flux. The contribution of the net heat flux through open and refrozen leads to the total net heat flux through sea ice is twice as large as the area contribution of open and refrozen leads to the total area covered with sea ice. In the eastern and central parts of the Weddell Sea, leads contribute some 30% to the total energy flux from the ocean to the atmosphere. This flux increases from 10--15 W m-2 in the eastern and central Weddell Sea regions to 30 W m-2 in the western part of the Weddell Sea, where leads contribute more than 80% of the total net energy transfer. The increase is mainly due to the colder and windier atmosphere in connection with the higher variability of the ice motion in the diurnal and semidiurnal band in the western Weddell Sea. The contribution of leads in winter ice formation exceeds 50% in the whole of the Weddell Sea. Monthly area-weighted ice growth is 10--15 cm in the east and up to 30 cm over the western continental shelf region. In the western part of the Weddell Sea, tidal and inertial motions in the diurnal and semidiurnal bands enhance lead formation, and thus contribute 7% to total net heat flux, 12% to ice formation, and 23% to the salt mass released during ice growth. The results are used to assess quantitatively the importance of leads to the interaction of the ocean, the sea ice, and the atmosphere. ¿ 2000 American Geophysical Union