Almost half of Wales's primary-age children missed more than two weeks of school last year.
Latest statistics show that 92,230 pupils between the ages of five and 10 - 46.1 per cent of the cohort - missed more than 10 school days in 2007- 08. And more than 20,000 pupils had over five weeks off.
The Assembly government said younger children were more prone to longer periods of illness.
But Professor Ken Reid, author of last year's National Behaviour and Attendance Review report, blamed parents for taking family holidays during term time.
Overall absenteeism in Wales's primaries has fallen - from 6.9 per cent of missed half-day sessions in 2006-07 to 6.7 per cent last year - but the rate of unauthorised absences has remained at 0.9 per cent.
Professor Reid expressed concern at the "significant differences" between local authorities, with unauthorised absences in rural areas generally much lower than those in large towns and cities.
In both Monmouthshire and Powys, for example, only 0.3 per cent of sessions were missed because of unauthorised absence, compared with 1.7 per cent in both Cardiff and Rhondda Cynon Taf.
But some areas have bucked the trend. In Swansea, unauthorised absences have fallen from 1 per cent in 2005-06 to 0.7 per cent last year, and in Neath Port Talbot, despite being one of the most deprived areas of Wales, unauthorised absences are among the lowest in Wales at 0.3 per cent.
Professor Reid said this highlighted the success of strong early intervention strategies, one of his main recommendations.
The education minister Jane Hutt said the Assembly government would look at the variations between local authorities and schools and ask them to consider their approaches to persistent absenteeism.
But there are no plans for Wales to publish a breakdown of the reasons for absenteeism, as England does. The Assembly government will publish its action plan on attendance next month.