Immunotherapy with intravenous recombinant human interleukin-2 (rh IL-2) may be accompanied by hypotension and the emergence of capillary leak syndrome. Nitric oxide (NO) is supposed to be responsible for both side effects. The aim of the current investigation was to elucidate the relationship between pro- and anti-inflammatory cytokines and the production of NO in eight tumor patients receiving intravenous rh IL-2 continuously over a time period of 120 hours. Markers of systemic inflammation, as well as nitrate plasma levels, were consecutively determined. Significant changes in the levels of pro-inflammatory cytokines IL-6 and IL-8 were observed (p < 0.05). In contrast to the anti-inflammatory cytokine IL-10, which did not increase significantly, the serum concentrations of the soluble tumor necrosis factor receptors (sTNFr) I and II rose continuously and significantly during the observation period (p < 0.05). In parallel, a significant rise in nitrate plasma levels was observed (p < 0.05). Moreover, there were highly significant correlations between nitrate and IL-6 serum levels (p < 0.05), nitrate and sTNFr-I (p < 0.05), nitrate and sTNFr-II (p < 0.05), and between IL-6 and IL-10 (p < 0.05), respectively. We conclude that immunotherapy with IL-2 promotes a pro-inflammatory state, parallelled by an increased production of nitric oxide. Although anti-inflammatory responses accompany this process, they are not able to diminish the production of nitric oxide.