This paper analyses ethical issues in forensic psychiatric research on mentally disordered offenders, especially those detained in the psychiatric treatment system. The idea of a 'dual role' dilemma afflicting forensic psychiatry is more complicated than acknowledged. Our suggestion acknowledges the good of criminal law and crime prevention as a part that should be balanced against familiar research ethical considerations. Research aiming at improvements of criminal justice and treatment is a societal priority, and the total benefit of studies has to be balanced against the risks for research subjects inferred by almost all systematic studies. Direct substantial risks must be balanced by health benefits, and normal informed consent requirements apply. When direct risks are slight, as in register-based epidemiology, lack of consent may be counter-balanced by special measures to protect integrity and the general benefit of better understanding of susceptibility, treatment and prevention. Special requirements on consent procedures in the forensic psychiatric context are suggested, and the issue of the relation between decision competence and legal accountability is found to be in need of further study. The major ethical hazard in forensic psychiatric research connects to the role of researchers as assessors and consultants in a society entertaining strong prejudices against mentally disordered offenders.