BACKGROUND To define the normal values of amniotic fluid alphafetoprotein in pregnant women, whose gestational ages range from 14 to 21 weeks, in the Hospital de Clínicas de Porto Alegre. METHODS One hundred thirty seven women with indication for amniocentesis were studied. The alphafetoprotein was measured in all samples using enzyme immunoassay. One hundred and nine normal pregnancies were selected. All of these fetuses had normal karyotype and had no malformation. They were not twins and their amniotic fluid samples were not bloody. These samples were divided by their gestational ages. Then the medians of the alphafetoprotein values and their multiples were calculated. RESULTS The medians of alphafetoprotein (KUI/ml) for each gestational age were as follows: 14 weeks: 16.32; 15 weeks: 14.36; 16 weeks: 13.43; 17 weeks: 10.93; 18 weeks: 8.22; 19 weeks: 7.35; 20 weeks: 5.62; 21 weeks: 4.47. CONCLUSIONS The establishment of alphafetoprotein normal values in our service allows us to use this assay for patients at risk of neural tube defects. It also makes possible to analyze samples sent for cytogenetic or metabolic studies, in order to identify elevated levels of alphafetoprotein, so that these fetuses could have a more detailed sonography study to look for malformations.