Background and Objectives: An overview of reviews is often necessary in order to compare relative effects of competing healthcare interventions, which is difficult if outcome measures are not consistently described among reviews. This paper aims to investigate the methodological difficulties in overviews due to inconsistent reporting of outcomes, using published Cochrane systematic reviews (CSRs) on different treatments for asthma. Methods: We searched the Cochrane Database of Systematic Reviews for reviews examining pharmacological interventions for asthma. Data on type of intervention, patients’ characteristics and outcome measures were extracted. We investigated outcome measures reported in five CSRs that compared the treatment effects of corticosteroid drugs. Results: The interventions involved pharmacological treatments for adults or children with asthma. Thirty-six different outcomes were reported in 64 CSRs (1228 trials). The most common outcomes were FEV1, PEF and withdrawal rates. Outcomes reported in five corticosteroid CSRs (284 trials) varied. Even for the same outcome measure the different review may employ different outcome statistics. For example, FEV1 was reported differently as baseline FEV1 (6 trials), endpoint FEV1 (51 trials), change in FEV1 from baseline (68 trials), percentage predicted at endpoint (29 trials), and change in percentage predicted (17 trials). Similarly, the measures of morning PEF included data on baseline (4 trials), endpoint (43 trials) and changes (81 trials); the measures of withdrawals included withdrawal due to asthma exacerbation (59 trials), or adverse events (50 trials), or lack of treatment efficacy (63 trials). No associations were found between trial design (parallel or crossover study) and the selection of outcome measures. Conclusions: Many different outcomes or endpoints had been chosen to assess the effects of asthma treatments within and between reviews. This makes it difficult to compare treatment outcomes from different trials and between reviews. Consistency in outcome reporting especially for the patient-focused outcomes in trials and reviews is pressingly needed.