Eighty-one of the 579 extracorporeal lithotripsies (ECL) performed between 1987 and 1992 with an ultrasound-guided hydroelectric lithotriptor required the use of another treatment modality. 53 percutaneous nephrolithotomies (PCNL) were performed in these 81 patients after failure of ECL (21 for non-fragmentation, 32 for residual fragments). The authors analyse the factors responsible for failure in order to select the more appropriate type of treatment for different types of stones. The site of the stones, their size, their radiographic appearance, their chemical composition, the number of ECL sessions and the number of days spent in hospital were compared between the PCNL and ECL groups. Smooth, homogeneous stones denser than bone and larger than 15 mm in diameter were significantly more resistant to ECL, despite an increased number of shock waves. PCNL was successful in 92% of cases of non-fragmented stones (with no residual fragments). The success rate for multiple residual fragments was 64%. In view of the risk of complications associated with residual fragments, regardless of their size, the potential risks of ECL and cost imperatives, the authors recommend that these stones, considered to be resistant, should be treated immediately by PCNL, as the results of PCNL after ECL are less satisfactory.