This paper reports the result of sewage pollution monitoring conducted in South and Southeast Asia during 1998-2003 using linear alkylbenzenes (LABs) as molecular tracers of sewage contamination. Eighty-nine water samples collected from Malaysia, Vietnam, and Japan (Tokyo), and 161 surface sediment samples collected from Tokyo, Thailand, Malaysia, Philippines, Vietnam, Cambodia, Indonesia, and India were analyzed for alkylbenzenes. The concentration range of ∑LABs in river water particles in Southeast Asia (<0.005-0.913μg/L) was comparable to or higher than those found in Tokyo (<0.005-0.638μg/L). I/E ratios (a ratio of internal to external isomers of LABs) in tropical Asian waters were close to the value of LABs in raw sewage (∼1) and much lower than those in secondary effluents (3-5). This suggests that untreated or inadequately treated sewage is discharged into the water. ∑LABs concentrations in sediments from South and Southeast Asia ranged from <0.002-42.6μg/g-dry with the highest concentration occurring at several populous cities. Low I/E ratios of the sediments with high ∑LABs concentrations suggest a heavy load of untreated sewage. Clearly in view of the current data and evidence of the implications of sewage pollution, this paper highlights the necessity of the continuation of water treatment system improvement in tropical Asia. © 2004 Elsevier Ltd. All rights reserved.