IntroductionThe purpose of the study is to describe arsonists in their demographic, psychiatric and criminological dimensions and to explore factors associated with the expert opinion of insanity. Material and methodsA series of cases accused of arson and assessed in the adult Psychiatric Unit of the Legal Medical Service of Chile, during the period 1999-2012 (N=197) is described. With a cases and controls design, the factors associated with the opinion of insanity are explored. ResultsThe series of cases had a high prevalence of mental disorders, social and family problems. Expert opinion compatible with the condition of criminal responsibility was achieved in 52.3% of the cases; and in 24.4% it was compatible with insanity and in 23.3% with diminished criminal responsibility. ConclusionsThe conditions of insanity and diminished criminal responsibility, grouped together as “expert opinion of altered criminal responsibility” were significantly associated with bizarre behavior, pathological motivation, emotional motivation, self-harm and cooperative attitude of the accused.