The purpose of this study was to evaluate the monitoring and diagnostic potential of MRI in fetal lung development and disease using lung volume and signal intensity changes through gestation. Thirty-five healthy fetuses (22-42 weeks) were examined on a 1.5- T MR system using sagittal T2w single-shot fast spin-echo imaging (TR indefinite, TE 90 ms, slice thickness/gap 3-5/0 mm, FOV 26-40 cm, NEX 0.5). Fetal body and lung were segmented manually and volumes calculated. Signal intensities (SI) of fetal lung and three reference values were measured on the section best displaying the lung. Regions of interests were defined by including the maximal organ area possible. The following SI ratios were generated: lung/liver, lung/amniotic fluid, lung/muscle, liver/fluid and liver/muscle. Volumes and ratios were correlated with gestational age. Data from seven fetuses with pulmonary pathology were compared with these normative values. Absolute lung volume varied from 12.3 to 143.5 cm(3) in correlation with gestational age ( P<0.001); lung volume relative to total body volume ranged from 1.6 to 5.0%, decreasing with gestational age ( P=0.001). All SI ratios measured were unrelated to gestational age. Diagnoses in the seven abnormal fetuses were hydrothorax ( n=2), congenital cystic adenomatoid malformation ( n=2), diaphragmatic hernia ( n=2) and pulmonary sequestration ( n=1); their absolute and relative lung volumes were below normal ( P<0.001). The SI ratios did not differ significantly from those in the normal population. Normative MR fetal lung volumes may have important clinical applications in confirming and quantifying intrauterine pulmonary hypoplasia and in complementing ultrasound in the planning of fetal and post-natal surgery. No clinical relevance was found for fetal lung SI values.