Patients with diabetes mellitus (DM) often suffer from various and severe infection. Patients with pancreatic DM have malnutrition due to chronic pancreatitis. Therefore it is believed that patients with pancreatic DM have a lower ability of host defence to infectious diseases than normal subjects. We examined the primpheral lymphocyte subsets (T cell, B cell, NK cell, CD3+ 56+ T) of eighteen patients (males, age = 52.8 +/- 12.7 years old, mean +/- SD) with pancreatic DM by two color flow-cytometry to evaluate the lymphocyte function. Ratios of T cell (T%, 66.9 +/- 9.3%, mean +/- SD). B cell (B%, 13.1 +/- 5.8%) and NK cell (Nk%, 19.3 +/- 8.7%) to total peripheral lymphocytes of patients with pancreatic DM were not significantly different from those (T% = 66.4 +/- 7.8%, B% = 13.5 +/- 6.7%, NK% = 19.9 +/- 9.1%) of thirteen normal subjects (males, age = 51.2 +/- 13.1). CD3+56+ T cell % (4.1 +/- 1.9%) of patients with pancreatic DM was lower than that (6.0 +/- 3.0%) of normal subjects (p < 0.05). CD3+56+ T cells have cytotoxic activity and it is likely that this activity is similar to that of NK cells. These results suggested that a decrease in peripheral CD3+56+ T cell % is a factor showing a weak host defense mechanism to infectious diseases in patients with pancreatic DM.