Technetium-99m sodium pertechnetate scintigraphy is an established method for evaluating organ perfusion and can be applied to the investigation of suspected testicular ischaemia due to acute torsion. One hundred and six consecutive scrotal investigations have been analysed qualitatively and quantitatively using a computer-based comparison of the perfusion slopes over each testis. Decreased vascularity on the symptomatic side only occurs in torsion and quantitative analysis is needed to detect it reliably. Increased vascularity on the symptomatic side can usually be detected qualitatively and occurs in orchitis, tumours, trauma and resolved torsion. Distinction between the causes of increased vascularity is aided by the clinical data. A halo of increased activity surrounding a relatively cold centre was seen in chronic torsion, some tumours and hydrocoeles. The test is both sensitive and specific for acute torsion but its usefulness is limited if it is not available at all hours. The test is useful, also, in confirming a diagnosis that does not require emergency operation and in identifying chronic torsion which is an indication for contralateral orchidopexy.