OBJECTIVE A novel whole blood platelet aggregometer has been developed. Based on differential light-scattering principles, the device detects platelet aggregates in undiluted blood. The primary objectives of this report are to introduce the design of this novel device and to evaluate its ability to assess platelet aggregation. METHODS In the light-scattering whole blood aggregometer, anticoagulated blood is dispensed into a self-contained sample loop and circulated with the help of a peristaltic pump. Laser light is directed into the circulating blood, and the scattered light signals are converted to electrical signals and analyzed. When platelet aggregates pass through the illuminated region, they produce peak signals, which are detected and quantified; the platelet aggregation profile is the time-dependent development of the total aggregate volume. RESULTS After the calibration of the light-scattering signals using polystyrene microspheres, platelet aggregation was measured by the light-scattering aggregometer, and the results were compared with those obtained with the conventional turbidometric (based on light transmission) and electrical impedance aggregometers. Scanning electron microscopy was used to confirm the presence of platelet aggregates in the whole blood samples taken from the light-scattering aggregometer. RESULTS Scanning electron micrographs and light-scattering observations confirmed that the peak signals detected after the addition of platelet aggregating reagents to blood were caused by platelet aggregates. Results of platelet aggregation and its inhibition in blood were similar to those obtained with the impedance whole blood aggregometer. CONCLUSIONS The novel light-scattering whole blood aggregometer has been shown to be a valid device to measure platelet aggregation and may be well suited for the assessment of platelet function in research and clinical blood samples.