The mass measles vaccination campaign was conducted in the slums of Surat City, in Gujarat State, as a part of urban measles control initiative in India. One dose each of the vaccine was administered to children in the age range of 9-59 months residing in these slums, regardless of their previous vaccination status.One year later, (October 2000), the present study was carried out in order to assess the impact of the mass vaccination campaign on the vaccination coverage and on the incidence of measles by comparing the findings with those of the baseline survey carried out in May 98. This was a retrospective study with a recall period of the preceding year. 3,147 children under five were studied in thirty slum clusters selected by the cluster sampling method. The parents/caretakers of these children were interviewed for information on any episode of fever with rash conforming to the case definition.The incidence rate for measles declined from 7.7 percent reported in the baseline (May 1998) to 3.5 percent in the impact assessment study. The incidence was 8 times higher in unvaccinated children. The mean and median age at contracting the illness increased from 26 +/- 14.2 months and 26 months in the baseline to 30.9 +/- 14.7 months and 30 months respectively in the impact assessment. The vaccination coverage had improved from 48.3 percent to 73.7 percent following the campaign.The compaign increased vaccination coverage decreased disease incidence and caused a shift towards higher age-groups in vaccinated children.