Digital blood flow was assessed by photoplethysmography, ultrasonic arterial flow tracing and measurement of systolic blood pressure, and compared to nailfold capillary microscopy in 20 normal controls, 16 patients with primary Raynaud's phenomenon, 40 with undifferentiated connective tissue disease and 30 with systemic sclerosis. All 4 measurements showed significant differences between controls, Raynaud's phenomenon and systemic sclerosis but only capillaroscopy (mean number of enlarged capillary loops and avascular score) was able to differentiate between primary Raynauds and undifferentiated connective tissue disease. Capillaroscopy (mean numbers of enlarged capillary loops) was the most sensitive (100%) and specific (81%) test with a positive predictive value of 90% for systemic sclerosis.