We used the nationwide Swedish Family-Cancer Database to analyse the risk for testicular cancer in offspring through parental and sibling probands. Among 0 to 70-year-old offspring, 4,586 patients had testicular cancer. Standardized incidence ratios for familial risk were 3.8-fold when a father and 7.6-fold when a brother had testicular cancer. Testicular cancer was associated with leukaemia, distal colon and kidney cancer, melanoma, connective tissue tumours and lung cancer in families. Non-seminoma was associated with maternal lung cancer but the risk was highest for the late-onset cases, providing no support to the theory of the in utero effect of maternal smoking on the son's risk of testicular cancer. However, the theory cannot be excluded but should be taken up for study when further data are available on maternal smoking. The high familial risk may be the product of shared childhood environment and heritable causes.