After Magreglio it flows through the Valassina and the comuni of Asso, Ponte Lambro and Erba, entering Lake Pusiano with the name of Lambrone.

At 5.8 cubic metres per second (200 cu ft/s) the average discharge of the Lambro is relatively small, but it can be occasionally boosted to 40 m3/s or more by the Milanese water drains and dangerous floods are frequent in the rainy seasons.

The name Lambro was used for a popular three-wheeler commercial transport vehicle produced by Milanese automaker Innocenti, which also made the Lambretta motor scooters.

Despite the implementation of sewage treatment, overall water quality remained poor, until the major disaster of 23 February 2010, when unknown criminals poured into the river, near Villasanta, the contents of several silos containing oil and other hydrocarbons, all belonging to a company named "Lombardia Petroli".

This disaster caused considerable damage to wildlife and vegetation, both in Lambro and in Po, and its effects will be evident for many years afterwards, making it one of the worst environmental crises in recent history in Italy.