This paper aims at investigating the validity of Lehman's Law of Increasing Complexity. Two metrics namely cyclomatic complexity and interface complexity were defined to capture increasing complexity. The goal was to verify if these metrics can be used to validate Lehman’s law of increasing complexity. Empirical analysis was performed using historical data collected on four evolving Open Source Systems (OSS). Results show that the considered Lehman’s law is partially supported by the collected data and the metrics. In particular, empirical results reveal that: total cyclomatic complexity and total interface complexity are increasing from version to version; average cyclomatic complexity and average interface complexity either declines or increases within a very short range; and function interface complexity hardly decline in evolving OSS. Also, the addition of low complex functions reduce cyclomatic complexity in evolving OSS but does little in reducing function interface complexity.