Considering that mutations in known prostate cancer (PrCa) predisposition genes, including those responsible for hereditary breast/ovarian cancer and Lynch syndromes, explain less than 5% of early-onset/familial PrCa, we have sequenced 94 genes associated with cancer predisposition using next generation sequencing (NGS) in a series of 121 PrCa patients. We found monoallelic truncating/functionally deleterious mutations in seven genes, including ATM and CHEK2, which have previously been associated with PrCa predisposition, and five new candidate PrCa associated genes involved in cancer predisposing recessive disorders, namely RAD51C, FANCD2, FANCI, CEP57 and RECQL4. Furthermore, using in silico pathogenicity prediction of missense variants among 18 genes associated with breast/ovarian cancer and/or Lynch syndrome, followed by KASP genotyping in 710 healthy controls, we identified “likely pathogenic” missense variants in ATM, BRIP1, CHEK2 and TP53. In conclusion, this study has identified putative PrCa predisposing germline mutations in 14.9% of early-onset/familial PrCa patients. Further data will be necessary to confirm the genetic heterogeneity of inherited PrCa predisposition hinted in this study. Prostate cancer (PrCa) is the most frequent cancer diagnosed in men worldwide, estimated to be responsible for the death of 27,540 men in the United States in 2015. Contrarily to other cancer types, the genetic contribution to the 10–20% of PrCa cases occurring in families with aggregation of the disease is largely unknown. Germline mutations in the BRCA2 and the MSH2 breast and colon cancer predisposing genes, respectively, explain only about 1.5% of our early-onset/familial PrCa cases. Taking advantage of recent deep sequencing technologies and an analysis pipeline established in our group, we have screened 121 PrCa patients with strong evidence of an hereditary component for mutations in 94 genes involved in several cancer predisposing syndromes. We found truncating/functionally deleterious mutations in seven genes and “likely pathogenic” missense variants in four genes, of which five and one, respectively, have not been previously associated with PrCa predisposition. We believe this study significantly contributes to the understanding of the genetic heterogeneity behind early-onset/familial PrCa.