The aim of this study is to analyze the effect of working capital management on firm profitability. In accordance with this aim, to consider statistically significant relationships between firm profitability and the components of cash conversion cycle at length, a sample consisting of Istanbul Stock Exchange (ISE) listed manufacturing firms for the period of 1998-2007 has been analysed under a multiple regression model. Empirical findings of the study show that accounts receivables period, inventory period and leverage affect firm profitability negatively; while growth (in sales) affects firm profitability positively.