Adenosine deaminase (ADA) activity, as a marker of cell-mediated immunity, was evaluated in the serum (S-ADA) and lymphocytes (L-ADA) of 47 children with idiopathic nephrotic syndrome, and 23 healthy controls. The mean S-ADA and L-ADA levels were significantly raised in active nephrotic syndrome (ANS) and in its sub-groups in comparison with controls. The ADA activity was significantly more elevated in relapsers than for the first attack of nephrotic patients, and the frequent relapsers had the highest enzymatic levels both in serum as well as lymphocytes. A significant positive correlation was found between serum and lymphocyte ADA levels (r =0.736, p <0.01). In remission, the S-ADA showed a significant fall in comparison with their corresponding ANS value (p <0.001) and reached the level of controls. The mean L-ADA also showed reduction but the difference was statistically insignificant and the value was significantly raised, when compared with controls. The enzyme activity in serum and lymphocytes normalized in the long-term remission group. Thus, ADA activity was abnormal in ANS cases, and L-ADA demonstrated change both in active as well as remission stage of the disease.