Floristic and phytogeographical information is presented in the first of three papers on the flora of Lesvos (East Aegean, Greece). The taxa included here are dicotyledons and belong to families in alphabetical order from Aceraceae to Guttiferae. Of the 92 taxa (79 species, 12 subspecies and one variety) presented, two are new for Greece, five are new for the Aegean islands as a whole, four are new for the East Aegean Islands and 13 are new for Lesvos. New distribution localities on the island are also given for some interesting taxa. Furthermore, the presence of 13 taxa, reported by previous authors without specific collection data, is confirmed. Critical comments are made on 22 taxa whose presence needs confirmation or is considered doubtful or wrong. Chorological, ecological and taxonomic comments are made for the more interesting cases and distribution maps for certain taxa in Greece or Lesvos are given. Finally, the threats and conservation status of some rare and interesting taxa are briefly discussed.