PURPOSE: Caveolin-1 (cav-1), the major protein component of caveolae, plays an important role in multiple signaling pathways, molecular transport, and cellular proliferation and differentiation. The specific functions of cav-1/caveolae are highly cell and context dependent. We have previously shown that cav-1 expression is increased in metastatic human prostate cancer and that cav-1 cellular protein expression is predictive of recurrence of the disease after radical prostatectomy. Recently, we reported that cav-1 is secreted by androgen-insensitive prostate cancer cells, and we detected, by Western blotting, cav-1 in the high-density lipoprotein(3) fraction of serum specimens from patients with prostate cancer. EXPERIMENTAL DESIGN: Using rabbit polyclonal antibodies with specificity for cav-1, we developed a direct sandwich immunoassay for the determination of cav-1 in serum. A recombinant human cav-1 fusion protein was overexpressed and purified from 293 PE cells and used as a calibrator. RESULTS: The assay was highly specific and had a minimum detection limit of 0.017 ng/ml (mean + 3 SD of zero calibrator) and measuring range of up to 200 ng/ml. Intra-assay coefficient of variation was 2.29-6.74% and inter-assay coefficient of variation was 2.81-6.43% over the serum concentration tested 0.04-31.89 ng/ml. The recovery limit of cav-1 by the assay was 89.55-100.28%. The median serum cav-1 level in 102 prostate cancer patients with clinically localized disease (0.463 ng/ml) was significantly higher than 81 healthy control men (0.324 ng/ml; P = 0.0446, Mann-Whitney test) or 107 men with benign prostatic hyperplasia (0.172 ng/ml; P = 0.0317, Mann-Whitney test). CONCLUSIONS: Our results indicate that serum cav-1 has the power to differentiate between prostate cancer and benign prostatic hyperplasia patients and the potential to be an important biomarker for prostate cancer. Additional studies to test the potential of serum cav-1 as a diagnostic and/or prognostic marker in prostate cancer are warranted.