More than 70 people may have caught coronavirus as a result of the Green Man Festival near Crickhowell.
The four-day event was attended by thousands of festival goers with no social distancing restrictions while the country was under 'Level Zero'.
Public Health Wales is carrying out active public health surveillance around large events.
A spokesperson said: "Analysis of contact tracing data gathered by Test, Trace, Protect has identified 71 Welsh residents with confirmed coronavirus, where Green Man festival was reported as a possible exposure in the seven days before showing symptoms.
"However, it is important to note that these cases are not necessarily attributable to transmission at the festival, as Green Man would have been one of a number of exposures reported by cases prior to onset, including, for example: socialising outside the festival, travel to and from the festival etc."
Ticket holders were required to show either an NHS Covid pass or a recent negative lateral flow test to enter.
In a statement, Fiona Stewart from Greenman Festival said: "There was a lot riding on this and we didn't want to let anyone down.
"From our post-festival questionnaire, the overwhelming response was that those attending felt we had done all we could to protect them from Covid-19 and that they felt safe, which was something that mattered a great deal to us."
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