Human pressures on the environment are changing spatially and temporally, with profound implications for the planet's biodiversity. Butterflies are important indicators for environmental change and are a suitable group to detect areas of high conservation concern and prioritize conservation efforts. To obtain data to support urgently-needed conservation measures, we surveyed the butterfly fauna in a mountainous region of northern Vietnam, using transect counts over 8 months (121 survey days) from June 2014 to April 2015. In total, we recorded > 26,000 butterflies belonging to 231 species, including at least two species new to Vietnam, three red-listed and protected species listed by CITES. Most species were rare: we recorded members of 100 species ae<currency>5 times and 52 species just once. Males dominated the sample, accounting for 81% of all observed butterflies and all members of 84 observed species. Species richness and abundance were highest in July, and there were surprisingly large changes in species composition between months. Species richness curves reached saturation, indicating that we detected most species present in the area, except for members of two families (Lycaenidae and Hesperiidae). Our results highlight the importance of thorough studies using standardized methods, capable of detecting most species in an area, over a whole season. There are urgent needs to integrate butterflies into conservation programs and use their potential as indicator species of habitat degradation and land use intensity.