Aim of this study was to investigate the use of upflow anaerobic solid-state (UASS) digestion for treating horse manure. Biochemical methane potential (BMP) tests conducted for varying mixtures of dung (hay and silage feed) and bedding material (wheat straw, flax, hemp, wood chips) showed that straw mixed with hay horse dung has the highest potential of 235.4 L-CH4 kg(vs)(-1). Continuous mesophilic digestion was conducted for 238 days using a single-stage UASS reactor (27 L) and a two-stage UASS system with an anaerobic filter (AF, 21 L). Increasing the organic loading rate (OLR) from 2.5 to 4.5 g(vs) L-1 d(-1) enhanced the methane rate of the single-stage reactor from 0.262 to 0.391 L L-1 d(-1) while the methane yield declined from 104.8 to 86.9 L kg(vs)(-1). The two-stage system showed similar yields. Thus, for solid-state digestion of horse manure a single-stage UASS reactor appears sufficient. (C) 2014 Elsevier Ltd. All rights reserved.