The accuracy of indirect oscillometric blood pressure measurements has been evaluated in 32 infants up to 11 months of age undergoing heart surgery. In a number of 1029 simultaneous measurements the indirect blood pressure was compared with the direct value obtained from a radial artery catheter. Cuffs of different sizes were applied. The main results were as follows: Regarding the ratio of cuff width to arm circumference, the best correlation between oscillometric and direct blood pressure measurements was obtained with a ratio of 0.38-0.41. The value of indirect blood pressure measurements depends on the absolute height of the blood pressure. In low blood pressure there is a tendency to underestimate and in high blood pressure there is a tendency to overestimate by the oscillometric blood pressure measurement. By applying an appropriate cuff size and by using our diagrams it should be possible to derive a direct value for the blood pressure on the basis of indirect oscillometric blood pressure measurements.