Inhibin is a potential tumour suppressor gene product in the gonads. While inhibin gene products may have a role in tumourigenesis, serum inhibin levels can be used as a marker for ovarian tumours derived from granulosa cells. Tumours derived from Sertoli cells, testicular counterparts of granulosa cells, are rare. To assess whether inhibin could be used as a human Sertoli cell tumour marker, serum inhibin and activin levels and inhibin subunit mRNA expression in the testis were studied. Northern blot and in situ hybridization revealed abundant expression of inhibin α, β_A, and β_B subunit mRNAs in large cell calcifying Sertoli cell tumours found in a 12‐year old boy with Carney complex. The tumours were multifocal and bilateral. Serum inhibin levels were clearly elevated at the time of the diagnosis, decreased by 50% after one of the testes was removed, and were low or undetectable after the second orchidectomy six weeks later. Activin was undetectable before the orchidectomies, while a low concentration of activin‐A was measured after them. Follicle stimulating hormone (FSH) concentration increased from normal pubertal value to castration level as expected. Normal seminiferous tubules also showed inhibin subunit α and β_B mRNA expression, whereas inhibin β_A mRNA was expressed in normal Leydig cells. These data suggest that serum inhibin reflects Sertoli cell activity and can be used as a human tumour marker.