The research phase of the National Suicide Prevention Project in Finland (from 1 April 1987 to 31 March 1988) included medico-legal investigation and psychological autopsy of all deaths suspected of being suicides, including 1397 official suicides and 61 undetermined deaths. In later analyses on suicide, undetermined cases were excluded. This paper presents an analysis of all officially classified undetermined deaths (n = 139) over the study period, consisting of all the initially suspected suicides (n = 61) and the remaining undetermined deaths (n = 78) where suicide could not be excluded. Poisoning by solids or liquids and drowning were the most common causes of all undetermined deaths. Suicidal intent was observed in 87% of undetermined deaths initially suspected of being suicides. In addition, 31% of these subjects had previously attempted suicide, and 34% had made suicidal threats. Depression was diagnosed in 23% of cases and alcohol dependence or abuse in 31% of cases. Undetermined deaths resembled suicides and appeared to reduce the suicide rate by 10%.