The search for new radiopharmaceuticals for tumour diagnosis usually proceeds on the basis of rational concepts drawing on the latest advances in molecular biology. Using this approach, radioactive peptide hormones, antibodies and oligonucleotides have been developed that are used increasingly in nuclear medicine for diagnostic and therapeutic purposes. This article, however, focusses on a group of radiopharmaceuticals whose use in tumour diagnosis was not the outcome of a methodical development programme but rather the result of a chance discovery. These radiopharmaceuticals, thallium-201 and technetium-99m labelled 2-methoxyisobutylisonitrile (MIBI), tetrofosmin and furifosmin, were first developed through extensive research efforts for cardiac imaging, but during their worldwide application for myocardial scintigraphy they were accidentally found to accumulate in tumours. Intensive studies were then begun on cell cultures in an attempt to discover the cause of their uptake into tumours. The aim was to compare the effectiveness of the radiopharmaceuticals for tumour diagnosis in a range of indications and to investigate the various mechanisms by which they are taken up into tumours. While the more favourable radiophysical properties of 99mTc-MIBI render it superior to 201Tl for many diagnostic purposes, neither 99mTc-tetrofosmin nor 99mTc-furifosmin has yet proved suitable for clinical routine examinations, although the former has found limited application. In the case of 99mTc complexes, the breakthrough came with the experimental finding that these substances are substrates of P-glycoprotein, a product of the human multidrug resistance gene (MDR1). The concentration of 99mTc complexes in tumour cells is a function of a passive, membrane potential-dependent influx into and a P-glycoprotein-controlled efflux out of the tumour cell. Preliminary studies suggest that in vivo detection of MDR may even be possible. There is also evidence that the P-glycoprotein-mediated transport system can be blocked competitively. However, it will be some time before a system can be developed for detection of MDR on a routine basis.