Overtly operated mobile speed cameras have been used in New Zealand since late 1993. Their operation has been confined to specific sites (called 'speed camera areas') which are mainly road sections with a record of speed-related crashes. A trial of hidden speed cameras began in mid-1997 in 100 km/h speed limit areas in one of New Zealand's four Police regions. This trial was still in progress at the time of writing and the current paper reports the results of an evaluation of the first year of the trial. During that period, the hidden cameras and related publicity were found (compared with the generally highly visible speed camera enforcement in the rest of New Zealand) to be associated with net falls in speeds, crashes and casualties both in speed camera areas and on 100 km/h speed limit roads generally. There were initial changes in public attitudes in response to the programme that later largely reverted to pre-trial levels. Compared with the localised effect of visible cameras on speeds and crashes mainly in speed camera areas, the hidden cameras had a more general effect on all roads. As further crash, speed and attitude data become available, the longer-term effects of the hidden camera programme will be evaluated.