The effect of vertical confinement on the natural convection flow and heat transfer around a horizontal heated cylinder is investigated. The flow characteristics and the time-resolved heat transfer have been measured respectively above and around the cylinder. It is shown that the primary effect of the vertical confinement is an increase in heat flux on the upper part of the cylinder for given separation distances between the cylinder and the fluid boundary. This increase is shown to be related to the large-scale oscillation of the thermal plume. The relationship between the flow pattern and the heat transfer characteristics at the cylinder surface is studied and the origin of the oscillation discussed. (C) 2003 Elsevier Ltd. All rights reserved.