Objective: Testicular maldescent is important because it is a common congenital disorder that is associated with an increased risk of infertility and testicular cancer. Murine studies indicate that testicular maldescent can result from disruption of insulin-like factor 3 (INSL3) activity and that it may be more severe when there is concurrent undermasculinisation. Therefore, the INSL3 gene was screened for mutations and polymorphisms that may contribute to testicular maldescent in patients with undermasculinisation as well as those with isolated testicular maldescent. Methods and Results: The patient groups consisted of individuals with isolated testicular maldescent n 28 and patients with undermasculinised genitalia and intra-abdominal n 24 or inguinal gonads n 33X The three control groups were: normal males n 15Y males with undermasculinised genitalia and scrotal gonads n 29 and females n 82X SSCP/HA mutation screening detected eight variants, five of which were predicted to alter the protein sequence (A-1G, V19L, P25S, A36T, R78H). Three of the amino acid changes (A-1G, V19L, R78H) each occurred in a single control sample and one was identified in a male with undermasculinised genitalia and intraabdominal testes (P25S). The A36T amino acid polymorphism was found in both patient and control groups at a similar frequency. Conclusions: The evidence suggests that INSL3 mutations and polymorphisms are not a major cause of testicular maldescent with or without associated undermasculinisation.