This depends on the tournament or club involved.

For International events, the ITTF has mandated use of the newer 40mm ball.

BUT... In the USA, the USATT has allowed local choice.

This means that in the USA you can use which ever ball you

and your playing partner(s) prefer EXCEPT for tournaments or leagues where the

decision is up to the organizers - who will (hopefully) follow local preferences.

--- Both balls are widely available from local or national suppliers.

At some clubs almost no-one has switched, at others nearly all have - usually

this depends on whether nearby tournaments are using 38mm or have switched to 40.

--- In Connecticut, more players in SW Connecticut are using 40mm, because some

of them attend monthly tournaments in New Jersey where Barry Dattel & Lilly Yip

have chosen 40mm.

--- But in Central and Northern Connecticut many are still using 38mm as are some

of the tournaments in the nearby Boston area.

--- The most common comment heard from those who've switched to 40mm seems to be

"It (40mm) isn't 'that bad' " though a few older players or those with vision

problems have stated a more enthusiastic acceptance of the slightly larger and

slower ball.