We studied plasma concentrations of thyroxine (T4), triiodothyronine (T3), free T4, free T3, thyrotropin (TSH), albumin and thyroxine-binding globulin (TBG) before and following 56 to 145 days (mean, 85) of refeeding in ten Indian patients who had severe protein-calorie malnutrition (PCM). The mean baseline plasma T4 concentration of 8.2 mug per 100 ml in these patients was comparable to the corresponding post-treatment value of 7.7 mug per 100 ml. However, since the dialyzable fraction of T4 (DFT4) was considerably higher (0.048 vs 0.029%), the mean baseline plasma free T4 concentration, 3.8 ng per 100 ml, was significantly greater than the mean post-treatment value of 2.2 ng per 100 ml. The mean baseline plasma concentration of T3, 21 ng per 100 ml, was markedly lower than the corresponding value of 96 ng per 100 ml after treatment. The mean plasma concentration of free T3, 94 pg per 100 ml, was also significantly lower than the post-treatment value of 303 pg per 100 ml. This was the case even when the mean DFT3 prior to treatment was significantly higher than that following treatment (0.46 vs 0.32%). The mean baseline ratio of plasma concentrations of total T3 and T4 (T3/T4 X 100) of 0.25 was significantly lower than the corresponding normal value of 1.3 after treatment. The mean plasma TSH concentration of 6.0 muU per ml in patients prior to treatment was comparable to the mean value of 5.5 muU per ml following treatment. The mean baseline plasma concentration of TBG of 3.3 mg per 100 ml was also comparable to the mean post-treatment value of 3.6 mg per 100 ml. The data on thyroid hormone levels in PCM can be explained if there were i) a selective increase in metabolic clearance rate of T3 without a change or a decrease in that of T4 and ii) a reversible defect in extrathyroidal conversion of T4 to T3. The latter possibility appears more likely.