During May and June 1990, a national mass measles immunisation campaign was undertaken in South Africa. This study is an assessment of the impact of the campaign on measles admissions to a provincial referral hospital that has specifically designated wards for children with communicable diseases. Data from the measles ward admissions book for the 18 months before the campaign (1 January 1989-30 June 1990) and 6 months after the campaign (1 July 1990-31 December 1990) were compared. Since the campaign, the average number of measles admissions has declined by 64.4% from 87 to 31 per month (P less than 0.01). Before the campaign, 21.3% of measles patients admitted were aged 7-9 months compared with 27.6% after the campaign, highlighting the urgent need to improve the measles vaccination coverage in this age group. An analysis of the geographical source of patients showed that measles continued to occur after the campaign in most of the areas where it existed before the campaign. It is concluded that important gains have been achieved by the campaign. These will be rapidly eroded and epidemics of measles may occur if measles vaccination efforts wane and slump back to pre-campaign levels. It is important to capitalise on the momentum generated through the campaign by continuing to support efforts of existing health care services to improve and maintain high levels of measles immunisation coverage.