Cerrado vegetation covers about 33% of the total area of the state of Piauí, northeastern Brazil, where there are also large areas of transition and contact with other vegetation types. Although the Piauí cerrados are a direct northern prolongation of the central core area, they are considered marginal by almost all authors. There are few previous records of their woody flora. This paper presents a floristic list of woody species based on the survey of 11 localities and updating of two already published lists. The climate belongs to Thornthwaite's subhumid category due to Piauí's position between the semiarid northeastern domain and the superhumid Amazon. The annual total rainfall is similar to that of the major part of the Brazilian cerrado area, but the rainy season is shorter. The soils differ from most cerrado soils in containing a greater proportion of concretions and plinthite with generally lighter colours, which may be interpreted as indicating large fluctuations of the water-table during pedogenesis. There are also widespread indications of seasonal high water-table showing that many of the Piauí cerrados probably belong to Sarmiento's hyperseasonal savannah category. A total of 60 families is represented by 307 woody taxa (including 14 family indet., four genera indet. and 80 species indet.). The lowest floristic diversity (richness) occurred in pure cerrado, the greatest in transition and contact areas.