We have developed a microfluidic immunoassay device using fiber optics to detect static light scattering (SLS) of latex microsphere agglutination. A 400-mum silica fiber was used to deliver blue light emitting diode (LED) or red laser light sources. A miniature, portable spectrometer was used to measure forward light scattering intensity collected by the same type of multi-mode fiber. To first show feasibility, anti-mouse IgG were used as target biomolecules and highly carboxylated polystyrene latex microspheres (510 nm) coated with mouse IgG were used as probes. Next, we tested for the vasculitis marker, anti-PR3, using the same type of microspheres coated with PR3 proteins. No false negatives or positives were observed. A limit of detection (LOD) of 50 ng mL(-1) was demonstrated for the vasculitis marker, anti-PR3. (Plasma samples from patients with vasculitis exhibited anti-PR3 at a median level of 380 ng mL(-1).) The optical detection system works without any fluorescence or chemiluminescence markers. The entire system proposed here is cost effective, small in size, and re-usable with simple rinsing. This may eventually lead to a portable, low-cost, re-useable, microfluidic, point of care immunoassay device.