Cidex OPA (0.55% ortho-phthalaldehyde) is marketed as a safer alternative to 2% glutaraldehyde for endoscope decontamination. As clinical experience is limited, an evaluation was undertaken in a busy endoscopy unit. Cidex OPA cycle dilution was monitored by manufacturer's test strips and high-pressure liquid chromatography (HPLC). Eight endoscopy staff completed daily occupational health questionnaires before and after its introduction. Patient throughput times were assessed in view of Cidex OPAs reduced disinfection time. HPLC confirmed that Cidex OPA levels are maintained above 0.3% for at least 50 cycles. Indicator strips proved generally reliable when tested by pharmacy staff. However, busy endoscopy staff found the indicator strips difficult to interpret, with 28 out of 223 (12.5%) test results being inappropriately recorded as 'fails'. Two hundred and two questionnaires were completed and no short-term health problems were noted. Apart from bronchoscopy lists, patient turnaround times were not improved. The increased cost of changing to Cidex OPA was estimated as pound 7691 per annum. Staining of washer-disinfectors was of concern as it proved very difficult to remove.