This study investigated the influence of airway secretion viscosity on the characteristics of crackle sounds produced using a mechanically ventilated porcine lung model. Aqueous ultrasonic methylene blue stained gel solutions of viscosity 100, 150 and 200 P were prepared and instilled into 15 isolated, mechanically ventilated, porcine lungs immersed in water. Sound signals recorded by a hydrophone before and after instillation of gel were subjected to both fast Fourier transform and wave-form analysis. At the completion of sound recording, the main bronchi were dissected and the location of the stained gel was photographically recorded. Wave-form analysis demonstrated that expiratory phase crackle incidence and amplitude were both significantly higher than inspiratory phase data. This study demonstrates that crackle duration and amplitude are inversely related to gel viscosity and that electronic lung sound analysis can provide indirect evidence of sputum viscosity.