Telecoms operator Safaricom on Monday said chief executive Bob Collymore had taken medical leave to receive specialised treatment for a "number of months".
Company chairman Nicholas Ng'ang'a made the announcement in a circular to employees, but did not disclose the nature of Mr Collymore's sickness.
"On behalf of the board, management and the entire Safaricom community, I wish Bob quick recovery and look forward to him resuming his duties as soon as the doctors allow him to do so," he said.
In his absence, Safaricom chief financial officer Sateesh Kamath will take a "primary role," the circular said, adding that he will be supported by Joseph Ogutu - the company's director of Strategy and Innovation.
The circular said Mr Ogutu has taken charge of Safaricom's day-to-day operations until Mr Collymore's return from medical leave. Mr Collymore, 59, a Guyanan-born British citizen, took charge of Safaricom in 2010 on a three-year contract, after the company's founding CEO, Michael Joseph, retired and had his term renewed for another three years.
His second term was expected to end last August, but was renewed for two more years.
The extension was seen as a pointer to the telco's preference for stability rather than change as it navigated a difficult operating environment characterised by ever changing regulations.
"You don't rock a boat when it is sailing smoothly and beautifully," said Mr Ng'ang'a.
Mr Collymore's time at the helm has seen Safaricom's revenues hit Sh212.9 billion in the year to March 2017 from Sh83.96 billion in 2010.
This has enabled Safaricom to cement its leadership in mobile money even as it continues to command market leadership in voice, mobile data and SMS services.
The sailing has, however, been far from smooth.
Mr Collymore took over during the heat of a price war among the different telecom operators and during his first investor briefing, he had the unenviable task of reporting a 13 per cent drop in profitability.
On Monday, Safaricom share price at the Nairobi Securities Exchange (NSE) remained unchanged at close of trading at Sh25.25 compared to Friday's price.
Safaricom's voice, data and M-Pesa business sustained more than 823,243 jobs in the year to March 2017, according to a study commissioned by the operator.
The jobs included those held by its own workers and the linkages created by products and services such as airtime dealers, mobile money agents and distributors.
More than 154,370 jobs were directly related to Safaricom's operations.
Official data released in April shows that the Kenyan economy created 832,900 new formal and informal sector jobs in 2016, down from 841,600 a year earlier, meaning Kenya's most profitable company sustained almost an equal number of jobs.