Background. Psychiatrists are asked to give opinions as to fitness to plead, a legal concept. There is a dearth of research into fitness to plead in the UK, with no prospective studies and no studies involving the comparison of fit and unfit subjects. In particular, there have been no investigations into the meaning of ‘unfit to plead’ in terms of psychiatric symptomatology, or as to the relative importance of each legal fitness criterion in psychiatrists' conclusions as to fitness.

Method. The study comprised a prospective evaluation of 479 consecutive referrals to psychiatrists at court. Individual legal fitness criteria were examined as predictors of unfitness. Associations of unfitness, and of individual legal fitness criteria, were examined with Brief Psychiatric Rating Scale (BPRS) symptom scores.

Results. The two most important of the legal criteria in clinical decisions as to unfitness were whether the person could follow the proceedings of the trial or give adequate instructions to their solicitor. The legal criteria concerning trial were more predictive of unfitness than those concerning plea. Unfitness was significantly associated with the presence of positive psychotic symptomatology, in particular conceptual disorganization and delusional thinking, but not with symptoms of anxiety, depression or withdrawal.

Conclusion. Unfitness is most significantly associated with symptoms affecting comprehension and communication. The fitness criteria could be simplified without loss of power. These results, predominantly concerned with mental illness, may not generalize to the mentally impaired.