The purpose was to analyze the influence of both mono-/dimeric X-ray contrast media (CM) and immunological co-factors on basophil degranulation. The study has been conducted in 31 adult patients who received nonionic CM injection for enhanced routine computed tomography examination. Plasma histamine, and basophil degranulation by using CD63 expression and flow-cytometry in blood samples of patients receiving iotrolan or iopromide injections were analyzed in vivo and in vitro (with different stimuli) before and up to 24 h after CM-injection. After iotrolan injection, histamine values remained unchanged and showed minimal increase after iopromide. In 5/12 patients receiving iopromide and in 5/19 in the iotrolan group histamine level rose (>100% of the baseline). After CM-injection, a significant activation of basophils (CD63) could be measured (P < 0.05). IL-3 prestimulus revealed a more pronounced CD63 expression after iopromide than after iotrolan. The IL-1β prestimulus seemed to be dose-dependent. Patients showing the most marked CM-induced CD63 expression had the highest predose values after in vitro N-formyl-methionyl-leucyl-phenylalanine treatment. Our finding suggests that increased histamine values and CD63 expression on basophils could depend on individual stimuli. Iopromide and iotrolan per se neither enhanced histamine release nor basophil degranulation under normal conditions. Analysis of CD63 expression by using fluorescence activated cell sorter-analysis is an interesting new tool to elucidate the complex conditions leading to CM-mediated hypersensitivity reactions.