Police in Wales say 10 registered sex offenders, two of whom are considered "high risk", are missing or unaccounted for.
The figures were disclosed to BBC Wales after a Freedom of Information request to the four police forces in Wales.
Seven offenders are missing in the South Wales Police force area and three in North Wales Police's jurisdiction.
The forces stressed the figures were only valid for the date the question was considered.
Registered sex offenders are required to inform police and probation officers of their address.
Of the seven missing in the South Wales force area (as of 15 November), two were considered high risk, four medium risk and one low risk.
One of the offenders has been missing for more than 10 years.
The force would not give details of the offences that had been committed by those missing.
Det Ch Insp Jane Mackay, of the public protection department of South Wales Police, said: "Registered sex offenders are managed very closely, which is evidenced by the low number of those in South Wales who have failed to comply with their conditions.
"On the rare occasions that these offenders fail to co-operate, the force takes a very robust stance in tracking them down.
"All cases are investigated thoroughly and are reviewed and monitored on a regular basis by senior investigating officers to ensure all lines of inquiry are pursued. This includes liaising with police services in other countries and Interpol.
"When these offenders are located we ensure that they are returned to the UK to face the consequences of their breach of licence, applications for extradition are therefore common practice."
Three registered sex offenders were missing in the North Wales Police area (as of 22 November), two of whom were considered medium risk and one low risk.
The offenders have been missing for a time period of between one month and five years.
The force would not give details of the offences that had been committed.
North Wales Police said it took a "pro-active approach in locating the very low number of RSOs (registered sex offenders) who fail to comply with their conditions".
"Monitoring officers conduct regular checks on any outstanding RSOs to ensure that any new information is acted on swiftly and shared with other law enforcement agencies," the force said.
"Additionally, probation officers are co-located with North Wales Police staff to ensure partnership working between agencies is streamlined, structured and cohesive."
All the forces said the figures were only valid for the date the question was considered because numbers would change as new information came to light.
Dyfed-Powys Police said no offenders were unaccounted for (as of 1 December), but two offenders had left the UK without giving notification.
Gwent Police said no offenders were missing (as of 15 November) although there had previously been a single case in 2010.
A spokesperson for the Victim Support charity said: "Victims and other members of the public need to feel safe and secure in their communities, and victims need to be reassured that they are not at risk of re-victimisation by seeing their offender on the streets they live in.
"These should be the most important considerations."
Mark Williams-Thomas, criminologist and former police detective, said: "It is clearly a concern that 10 registered sex offenders are unaccounted for in Wales, especially given that two are high risk.
"Knowing how sex offenders operate it may well be that they are currently in contact with children or vulnerable people, and therefore I would urge the police to release the details of the 10 offenders, in order that they can be located and any risk eliminated."