Animal experiments showed that elevated interstitial fluid pressure (IFP) is associated with poor blood supply and inadequate supply of drugs to solid tumours. IFP is approximately 0 mmHg in most normal tissues. Up to now there have been only few studies showing elevated interstitial pressure in human tumours in situ: Mammary carcinomas, cervical carcinomas and colorectal cancers have an elevated IFP. We measured IFP in squamous cell carcinomas of the head and neck region in humans using the ''wick-in-needle'' technique. In all lesions (n=25), the IFP was elevated (4-39 mmHg). The IFP increased with tumour size. The highest IFP was 39 mmHg in a 26 ml tumour. These results show that squamous cell carcinomas of the head and neck region in humans have an elevated interstitial fluid pressure. The elevation of IFP associated with inadequate delivery of drugs to the interstitium of malign tumours may reduce the response to therapy.