The effect of aspect ratio on heat transfer to a cylinder in cross flow of air has been studied experimentally. The aspect ratios tested were AR = 12, 10.8, 8.5 and 6 for a Reynolds number range of 6300 < Re-D > 50,960. The tunnel blockage ratio was 8.6%, and the cylinder had a uniform wall temperature. Local heat transfer coefficients were measured at various positions along the cylinders. Heat transfer rates at the centerplane on the rear of the cylinder increased with decreasing aspect ratio. At AR = 12 there was also a marked increase along the cylinder as the wall was approached: at AR = 6 the heat transfer was uniformly high along the cylinder. Consequently, circumferentially averaged and total averaged Nusselt numbers increased with decreasing aspect ratio. For example, on the centerline at Re-D = 33, 740, Nu(180degrees)/Nu(0degrees) was 44% higher for AR = 6 than for AR = 12, and circumferentially averaged values were 14% higher. Correlations for the averaged Nusselt numbers are presented that account for the effects of aspect ratio, tunnel blockage and free stream turbulence. (C) 2003 Elsevier Ltd. All rights reserved.