As part of a national vaccination experiment, fifteen herds in the province of Friesland had their calves vaccinated since 1984. In these herds, cows born before or in 1983 served as control. These cows were not vacinated in twelve herds or vaccinated at old age in three herds. Clinical disease declined dramatically in the experimental group. From 2,069 control cows of which post-mortem examination was done, 268 (13%) had developed clinical signs of Johne's disease compared to only 13 (1%) out of 1,140 experimental vaccinated cows. Microscopic and histologic examination of the organs revealed less difference between groups with respectively 27% positive findings in control cows compared to 11% in experimental vaccinated cows. Culturing the material for the presence of M. paratuberculosis showed 31% positive findings in control cows compared to 38% in experimental vaccinated cows.