The use of 4 Fr catheters for diagnostic coronary arteriography improves patient comfort and could potentially reduce costs by decreasing arterial complications and encouraging outpatient examination. We examined the performance of recently developed 4 Fr catheters (100 cm long, 0.042 inch internal diameter) through in vitro flow rate patterns and in vivo image quality. Flow patterns (flow rate calculated from pressure with the Hagen Poiseuille relation) were obtained by hand (10 ml volume; maximal effort, one-handed full injection) and power injection (10 ml volume; 5 ml/sec) in the 4 Fr catheters were compared to 5 Fr (0.047 inch) and 6 Fr (0.057 inch) catheters. In 20 patients, coronary arteriograms were obtained from 4 Fr hand injection (4 HI), 4 Fr power injection (4 PI), 6 Fr hand injection (6 HI ), and 6 Fr power injection (6 PI) were compared after being scored into 5 grades. Coronary diameter was assessed at two sites for all injections in the same group of patients. In power injections, peak flow rates were consistently 5.5 ml/sec in all catheters. Hand injections were much more variable; peak flow rate ranges for 5 different physicians were 3.5-6.0 ml/sec. in 6 Fr, 1.8-3.0 ml/sec. in 5 Fr and 1.4-2.0 ml/sec in 4 Fr catheters. Mean image scores of left arteriograms were significantly higher for 4 PI versus 4 HI and for 6 Fr images compared to 4 PI (p < 0.001). In right arteriograms, 4 HI scored significantly lower than other methods (4 PI, 6 HI and 6 PI; p < 0.001). In 4 HI left coronary arteriograms, the frequency of grade > 2 was 96% and > 3 was 82%. Power injection improved 4 Fr imaging to 99% greater than grade 2 and 93% greater than grade 3. There were no differences in the frequency of images above grade 3 in right coronary arteriograms. Measured coronary artery diameters at 40 normal and 17 stenotic sites by QCA analysis (Acom, Siemens AZ, Erlangen, Germany) did not influence the study data. We concluded that coronary arteriograms by hand injection with 4 Fr catheters were inferior compared to 6 Fr images. Power injection and/or larger catheters may be necessary to ensure the highest quality images are obtained.