Although ketoconazole has been used extensively in dogs for the treatment of various fungal infections, information about adverse effects is mainly anecdotal. Common adverse effects in humans include dose-dependant anorexia, nausea and vomiting, allergic rashes and pruritus. Drug-induced hepatitis is very rare, but potentially fatal. The aim of this study was to evaluate the type and frequency of adverse effects associated with ketoconazole therapy in dogs treated for skin diseases and any possible influence of dosage, duration of therapy, signalment or concurrent medication. The medical records of 632 dogs treated with ketoconazole (2.6–33.4 mg/kg) were reviewed. Adverse effects occurred in 14.6% (92 dogs) and included vomiting (7.1%), anorexia (4.9%), lethargy (1.9%), diarrhea (1.1%), pruritus (0.6%), erythema (0.3%) and other adverse effects (2.5%). Of the dogs with other adverse effects, four of 16 (25%) were ataxic and three of these received concurrent ivermectin. Adverse effects were significantly more often recorded in dogs concurrently treated with ciclosporin (P = 0.034) or ivermectin (P = 0.007). Increased liver enzyme levels were reported rarely, and icterus was not seen in any of the dogs. However, monitoring liver enzymes during therapy is recommended, although this might not necessarily prevent severe idiosyncratic hepatotoxicity.