Sources of error theoretically possible in the registration of suicide are discussed. Using cause-of-death statistics for each of the Nordic countries, the relative influence of the various sources of error is evaluated. Two types of sources of error are identified, one is when cause-of-death is registered as drowning and the other is when it is registered as "unknown". Results have shown however, that the low probability of suicide in these cases, and consequently small numbers involved, could hardly influence differences in rates between the Nordic countries. The differences found in suicide statistics therefore reflect true differences in the frequency of suicide between these countries.