Heat transfer of a liquid to a solid wall in natural convection is studied in the regime of laminar to turbulent transition in the presence of longitudinal vortices. These vortices lead to an increase in heat transfer, especially in their later stage before breakdown. This last increase is significant and even higher than one would expect for a turbulent flow. In this work heat transfer was studied numerically and in experiments using thermochromic liquid crystals. (C) 2003 Elsevier Science Ltd. All rights reserved.