The relationship between three amniotic fluid parameters (fat staining cells, anucleate cells and creatinine) and gestational age has been studied in 166 samples from 107 patients of known gestational age. Individually there is an unacceptable degree of inaccuracy. However, when the three parameters are combined in a multilinear regression, an equation for gestational age is produced that is reliable to with +/-14 days in over 90% of cases. This held true when the equation was applied to 60 samples from 45 patients with uncertain dates and related to the paediatric estimate of gestational age. The accuracy was unaltered by intrauterine growth retardation.