This paper presents art empirical correlation for predicting the critical heat flux (CHF) of vertical, upward, steam-water flows in round tubes for low and medium Pressures Under circumferentially non-uniform heating conditions. This correlation is based on experiments carried out with test sections having an inner diameter of 22 mm and heated lengths ranging from 1.8 to 3.5 m. The ratios between the maximum and the minimum heat fluxes were 1.0, 4.7 and 8.3. The experiments were carried out for outlet pressures and mass fluxes ranging from 10 to 40 bar and 300 to 1600 kg m(-2) s(-1) respectively. The root mean sqare error of predicted CHF values by using the proposed correlation and applying the heat balance method is 3.7%. (C) 2007 Elsevier Ltd. All rights reserved.