The incidence of INH-associated liver injury was evaluated in 239 children aged between 9 and 14 years, who were receiving 300 mg INH/day for tuberculosis prophylaxis. Serum SGOT and SGPT levels were determined before INH administration and at 4-weekly intervals thereafter. Levels of both enzymes were raised during the first 3 months of treatment in 18 (7.5%) children, while in 23 (9.6%) children either SGOT or SGPT exceeded normal levels (SGOT greater than 40 units, SGPT greater than 30 units). Only 2 (0.8%) children showed SGOT and SGPT values above 100 units and in them treatment with INH had to be discontinued. In all other children transaminases returned to normal during uninterrupted INH administration. It was noted also that transaminase values in children who did not exhibit a rise above normal, still had significantly higher levels during treatment compared with before. The findings of this study suggest that liver injury in children receiving INH for prophylaxis occurs more often than it had hitherto been believed but that it is usually mild and transient.