Revenue deserves more attention in mergers; indeed, a failure to focus on this important factor may explain why so many mergers don’t pay off. Too many companies lose their revenue momentum as they concentrate on cost synergies or fail to focus on postmerger growth in a systematic manner. Yet in the end, halted growth hurts the market performance of a company far more than does a failure to nail costs. Some balance may have to be restored.

The belief that mergers drive revenue growth could be a myth. A Southern Methodist University (SMU) study of 193 mergers, worth $100 million or more, from 1990 to 1997 found that revenue growth was fairly elusive. Measured against industry peers, only 36 percent of the targets maintained their revenue growth in the first quarter after the merger announcement. By the third quarter, only 11 percent had avoided a slowdown; the median lag was 12 percent. When McKinsey joined the SMU researchers to take a closer look, it turned out that the targets’ continuing underperformance explained only half of the slowdown; unsettled customers and distracted staff explained the rest.

Moreover, these revenue shortfalls don’t represent the beginnings of a J-curve. Further McKinsey research...