Factors related to insulin resistance have been implicated in prostate cancer development, however, few analytical studies support such an association. We performed a case control study on 392 prostate cancer cases and 392 matched controls nested in a prospective cohort in Northern Sweden. Plasma concentrations of C-peptide, leptin, glycated haemoglobin (HbA1c) and fasting and post-load glucose were analysed and homeostatic model assessment of insulin resistance (HOMA-IR) was calculated. Conditional logistic regression analyses were used to calculate odds ratios (OR) of prostate cancer. High levels of C-peptide, HOMA-IR, leptin and HbA1c were associated with significant decreases in risk of prostate cancer, with ORs for top vs. bottom quartile for C-peptide of 0.59 (95% Confidence Interval [CI], 0.40-0.89; p(trend) = 0.008), HOMA-IR 0.60 (95% CI, 0.38-0.94; p(trend) = 0.03), leptin 0.55 (95% CI, 0.36-0.84; p(trend) = 0.006) and HbA1c 0.56 (95% CI, 0.35-0.91; p(trend) = 0.02). All studied factors were strongly inversely related to risk among men less than 59 years of age at blood sampling, but not among older men, with a significant heterogeneity between the groups for leptin (p(heterogeneity) = 0.006) and fasting glucose (p(heterogeneity) = 0.03). C-peptide and HOMA-IR were strongly inversely related to non-aggressive cancer but were non-significantly positively related to risk of aggressive disease (p(heterogeneity) = 0.007 and 0.01, respectively). Our data suggest that androgens, which are inversely associated with insulin resistance, are important in the early prostate cancer development, whereas insulin resistance related factors may be important for tumour progression.