Objective To assess the diagnostic and clinical contribution of in utero magnetic resonance (iuMR) imaging in fetuses diagnosed with abnormalities of the posterior fossa as the only intracranial abnormality recognised on antenatal ultrasonography (USS). Methods We report a sub-group analysis of fetuses with abnormalities of the posterior fossa diagnosed on antenatal USS (with or without ventriculomegaly) from the MERIDIAN cohort who had iuMR imaging within 2 weeks of USS and outcome reference data were available. The diagnostic accuracy of USS and iuMR are reported as well as indicators of diagnostic confidence and effects on prognosis and clinical management. Results Abnormalities confined to the posterior fossa according to USS were found in 81 fetuses (67 with parenchymal and 14 with CSF-containing lesions). The overall diagnostic accuracy for detecting an isolated posterior fossa abnormality was 65% for USS and 88% for iuMR (difference = 22%, 95% CI: 14.0 to 30.5%, p < 0.0001). There was an improvement in ‘appropriate’ diagnostic confidence as assessed by a score-based weighted average’ method (p < 0.0001) and a three-fold reduction in ‘high confidence but incorrect diagnoses’ was achieved by using iuMR imaging. The prognostic information given to the women after iuMR imaging changed in 44% of cases and the overall effect of iuMR on clinical management was considered to be ‘significant’, ‘major’ or ‘decisive’ in 35% of cases. Conclusions Our data suggests that any woman whose fetus has a posterior fossa abnormality as the only intracranial finding on USS should have iuMR imaging for further evaluation. This is on the basis of improved diagnostic accuracy and confidence which has substantial effects on the prognostic information given to women and changes in clinical management.