In the past mucoactive drugs in airway diseases have been identified and profiled in symptom-based animal experiments and in clinical trials along related lines (cough and expectoration). Presently available drugs of this class are not generally accepted by licensing authorities worldwide and no new molecule clinically profiled as a mucoactive drug has been brought to regulatory approval in the past 20 years. Among regulatory guidelines only the CPMP 1999 'points to consider' on drug development in chronic obstructive pulmonary disease (COPD) advises for mucoactive drug development by suggesting that an indication for symptomatic treatment may be established on the basis of a symptom-related primary endpoint that should be justified as for its importance and supported by a co-primary lung function endpoint. Quality and safety of the new drug must be documented in long-term studies and the indication and use clearly described based on established or adequately profiled new primary endpoints in two pivotal studies. Published trials on mucoactive drugs have used a variety of endpoints. These include mucus hypersecretion-related symptoms by questionnaire, expectorated volume and dry weight, and mucus viscosity, elasticity and transportability. Most methods and endpoints are not validated and a positive standard of treatment is not established. New surrogate markers of efficacy for shorter term trials, e.g. induced or spontaneous sputum based assays (cellularity, mucus antigens), exhaled breath (NO), breath condensate (eicosanoids) or airway biopsy are only partially validated and the risk of false positive or negative phase II results is appreciably high. On the other hand, lung function measurements including airway hyper-reactivity assessment and typical phase III (long-term) endpoints like dyspnoea ratings, health status assessments, incidence of exacerbations and lung function decline over time are validated endpoints and offer a high likelihood of regulatory acceptance. Proof for no depression of lung mucociliary clearance is an important safety endpoint.