The purpose of this paper is to analyse disclosure of environmental information (DEI) for a set of 100 companies listed at the Bucharest Stock Exchange (BSE) and identify possible correlations between this and the evolution of some relevant economic and financial measures of companies’ activities. For these purposes we have calculated an index of environmental information disclosure and we employed a system dynamic panel data estimation model and panel corrected standard errors for sampled companies for the 2013–2017 period. The results we have obtained show that sampled companies have a low degree of environmental information disclosure, as the highest registered score was of 15 out of a maximum of 29 points, with an average of merely 6.37 points. Regarding the possible correlations, the tests performed have shown that entity size, expressed by the number of employees, is the factor which positively influences environmental information’s disclosure. Results also evidence that performance determines the quantity of information the firm provides to external users, as opposed to maturity/age. Our study is the first approaching companies from Bucharest Stock Exchange with data for 5 years using a mixed approach (DEI–index and regressions) and we think the results obtained are useful for managers, general public and investors, considering that size and performance greatly influence companies’ environmental awareness.