Workshops to identify transboundary and basin-wide environmental issues and a diagnostic study by consultants identified priority environmental concerns of resource managers in the lower Mekong River basin. The issues identified, in priority order, were water quality, reduction in dry season flows, sedimentation, fisheries decline, wetland degradation, and flooding. An analysis of the available data found no evidence that water quality was poor except in the delta, where nutrient levels were high and increasing. Dry season flows have not decreased, and in the immediate future they are more likely to increase. Suspended sediment levels in the river are not high, and there is no indication that sediment loads are substantially increasing. Fish catch per unit effort has declined over the past decades, as have catches of large fish, but total fish catch has increased. Flooding does not appear to have increased in frequency or extent. There is no reliable quantitative information available on changes in wetland extent or condition, although it is reasonable to assume that both have declined. Reasons for the mismatch between perceptions and the data may include a failure by management agencies to analyze and publish data and provide adequate responses to issues raised in the popular press. This results from a lack of capacity in many government agencies and the Mekong River Commission, where there are high staff turnover rates and a dependence on short-term experts with limited experience in the basin.