This study was done to assess the validity of nontriggered 2D phase contrast MR angiography for measuring blood flow in human arteries and veins. Volume in the popliteal and internal carotid arteries was measured by nontriggered and triggered 2DPC in 1.3 normal volunteer (mean age 27, range 18-46). Parameter selection was guided by previous phantom experiments. Results were compared by linear regression analysis. Measurement error was determined by one-way analysis of variance of repeated measurements. In the internal carotid arteries, good agreement was found between the volume flow, Q, as determined by a triggered measurement and a nontriggered measurement: Qntr = 0.988 (+/- 0.006) Qtr, r = 0.98, SEE = 0.16 ml/s. The estimated measurement errors of both techniques were of the same order: 0.27 vs. 0.31 ml/s. Substantial deviations between triggered and nontriggered 2DPC were found in the popliteal artery: Qntr = 0.827 (+/- 0.028) Qtr, r = 0.97, SEE = 0.12 ml/s. The estimated measurement error of nontriggered 2DPC turned out to be twice as large as of triggered 2DPC here: 0.22 vs. 0.13 ml/s. We believe that nontriggered 2DPC is a valid technique for measuring blood flow in stationary vessels with weakly pulsatile flow, but merely provides a rough estimation for strongly pulsatile flow. In its current implementation, nontriggered 2DPC provides the data in 40 s, whereas triggered 2DPC requires 3-4 min, and offers additional time savings with regard to patient preparation and data processing.