Summary A case-control study of the aetiology of testicular cancer was conducted using information obtained by interview and from case-notes of 259 cases with testicular cancer and two sets of control patients -238 men with diagnoses other than testicular cancer attending the same radiotherapy centres as those attended by the cases, and 251 hospital in-patients not attending radiotherapy departments. Logistic regrcssion analyses were performed, after stratifying by age and region of residence, to estimate the relative risks (RRs) associated with various aspects of prior medical history. The risk of testicular cancer was found to be raised for men with a history of cryptorchidism (RR based on comparison with all controls =6.3; P<0.001), inguinal hernia (RR= 1.6; P=0.14), mumps orchitis (RR= 12.7; P=0.006), atopy (RR= 1.8; P=0.03), and meningitis (RR=3.0; P=0.21). Inguinal herniorrhaphy before the age of 15 years was particularly a risk factor for seminoma, whereas the relative risks were similar for seminoma and teratoma for the other factors. The results add to the growing evidence that congenital abnormalities involving the process of testicular descent and closure of the processus vaginalis are risk factors for testicular cancer, and that some types of testicular damage later in life may also be important. The findings of associations with previous atopy and certain infections suggest a possible second aetiological mechanism -that immunological abnormalities may be associated with an increased risk of testis cancer.Testicular cancer is one of the commonest cancers in young men and is increasing in incidence in adults in many white populations, but its aetiology remains largely unknown. There is growing evidence of an aetiological role for prenatal factors (Henderson et al., 1979;Loughlin et al., 1980;Schottenfeld et al., 1980;Depue et al., 1983), and testis cancer is associated with some congenital malformations involving abnormalities of genito-urinary tract development. Associations have been shown with cryptorchidism, which is found in about 10% of eases (Morrison, 1976;Henderson et al., 1979;Schottenfeld et al., 1980;Depue et al., 1983;Mills et al., 1984;Pottern et al., 1985), inguinal hernia (Li & Fraumeni, 1972;Morrison, 1976;Swerdlow et al., 1982;Depue et al., 1983) and, for childhood testicular cancer, congenital genito-urinary abnormalities other than cryptorchidism (Li and Fraumeni, 1972;Sakashita et al., 1980;Swerdlow et al., 1982). Adult, but not childhood, testicular cancer incidence has increased in white populations in recent years which suggests that postnatal factors may also be important in the aetiology of the adult disease. Mumps orchitis appears to be an uncommon postnatal cause of the tumour (Beard et al., 1977a; Lin and Kessler, 1979), but there has been little investigation of postnatal disease associations.The present study examined associations between various previous diseases and testicular cancer in data collected in a case-control study conducted in the environs of Oxford and the ...