Adenosine deaminase is one of the key enzymes in purine nucleotide degradation. This enzyme exists in most of the human tissues and the activity is high in lymphatic tissues, especially in T lymphocytes. Elevated adenosine deaminase activity in T cell leukemia has been reported, and its inhibitor, deoxycoformycin, has been developed as an antitumor agent. In some types of leukemia, serum adenosine deaminase activity increases in accordance with the severity of the disease. Although mycosis fungoides rarely involves peripheral blood, tumor cells do invade the skin. In order to evaluate the clinical significance of adenosine deaminase in mycosis fungoides, adenosine deaminase activity was measured in sera of 15 patients with mycosis fungoides at various stages. The mean enzyme activity was 23.2 IU/l, which was high with statistical significance compared with healthy controls (P < 0.001). Nine of twelve patients in the plaque stage (T2N0M0, IB) showed higher adenosine deaminase activity than did the normal population. The mean adenosine deaminase activity in sera in the patients in the plaque stage (T2N0M0, IB) was as high as 19.0 IU/l (range 13.7-21.4) with statistical significance compared with healthy control (P < 0.001). Three tumor stage patients without visceral involvement (T3N0M0, IIB) showed higher levels of adenosine deaminase activity (19.7, 21.5, 24.4 IU/l). An erythrodermic patient (T4N0M0, III) also had a high adenosine deaminase activity 28.4 IU/l. Two tumor stage patients with organ involvement (T3N0M1, IVB) exhibited extremely high adenosine deaminase activity (60.9, 32.2 IU/l). The adenosine deaminase activity in sera showed a tendency to become higher with the extension of the stages.(ABSTRACT TRUNCATED AT 250 WORDS)