The effect of raising the temperature of the irrigant solution on the smear layer was evaluated in the middle and apical third of 22 human upper incisors. A 5% sodium hypochlorite (NaOCl) solution was used at 21 degrees C and at 50 degrees C. After hand instrumentation and treatment with the irrigant, teeth were fractured into halves and examined by scanning electron microscopy. Characteristics of the smear layer in the two groups of specimens were compared. In the middle third, where NaOCl had been used at 50 degrees C, the smear layer was thinner and made of finer, less well-organized particles than where it had been used at 21 degrees C. In the apical third, the smear layer was of almost the same thickness in the two groups of specimens, although the particles were finer where the NaOCl had been used at 50 degrees C.