A total of 2711 pulmonary artery wedge pressure (WP) measurement attempts were made prospectively from WP recordings in 44 (30 men) critically ill patients, using 77 flow-directed catheters. Of these, 322 (12%) failed to yield a WP measurement, and 521 (18%) were associated with technical problems. One half of these technical problems were due to poor dynamic response or damped pressure tracings; other problems included balloon overinflation, partial WP, and inability to aspirate blood from the pulmonary artery (PA) port. Only 50% of wedge blood sampled at the time of initial PA catheterization yielded capillary blood (PO2 greater than or equal to 10 torr higher than PaO2). In 12 stable patients in whom paired measurements were available, there were clinically important differences (-13 to +22 torr) between paired WP measurements made before and after rapid correction of technical problems. Technical problems are common and may be associated with clinically important errors. Those due to poor dynamic response are easily and rapidly detected at the bedside.