To enable use of Empididae s.l. (Diptera) as a tool in nature conservation, a Red Data Book of this taxonomical group was generated for Flanders, Belgium. All distribution data on species in Belgium between 1887 and 1999 were gathered from collections as well as personal sampling efforts by the first two authors. This resulted in about 21,000 records of Empididae, Hybotidae, Microphoridae and Atelestidae with 16,119 records for Flanders (northern Belgium) and 4776 for Wallony (southern Belgium). All species were assigned to Red Data Book categories which are based on a combination of a rarity and a trend criterion. Rarity is expressed as the proportion of the total number of UTM 5 km squares sampled in which the species have been found since 1981. The trend criterion is interpreted as the change of the species rarity between 1887-1980 and 1981-1999. A comparable number of UTM 5 km squares was investigated during the two time periods. A total of 259 species were recorded in Flanders. Twenty-seven or 10\% of them are considered as 'extinct in Flanders', 10 (4\%) as 'critically endangered', 12 (5\%) as 'endangered', 11 (4\%) as 'vulnerable', 99 species (38\%) as 'susceptible' or 'rare', 65 species (25\%) as 'safe' or 'at low risk' and 34 species (13\%) are assigned to the category 'data deficient' due to taxonomic problems or a lack of ecological data. Only one of the common species shows a recent decrease of more than 50\% and is classified as 'nearly threatened'. Current threats in most species are related to the alteration or destruction of their favoured habitats. The results are discussed in the light of recent criticism of the use of Red Data Books in nature conservation.