GAAP operating expenses for first quarter 2018 were $71.7 million, or 26.1 percent of revenue, and $64.7 million, or 23.6 percent of revenue, on a non-GAAP basis, which excluded $4.8 million of amortization of acquisition-related intangible asset expenses, $2.6 million of expenses related to officer retirement and a credit of $0.3 million related to the KFAB restructuring. GAAP operating expenses in the first quarter 2017 were $64.6 million, or 27.3 percent of revenue, and in the fourth quarter 2017 were $72.9 million, or 27.2 percent of revenue.

First quarter 2018 GAAP net income was $18.5 million, or $0.37 per diluted share, compared to net income of $1.2 million, or $0.02 per diluted share, in first quarter 2017 and a net loss of ($30.7) million, or ($0.62) per share, in fourth quarter 2017 which includes $45.9 million tax expense associated with the tax law reform.

First quarter 2018 non-GAAP adjusted net income was $24.2 million, or $0.48 per diluted share, which excluded, net of tax, $3.9 million of non-cash acquisition-related intangible asset amortization costs and $2.0 million of officer retirement expenses. This compares to non-GAAP adjusted net income of $7.0 million, or $0.14 per diluted share, in the first quarter 2017 and $21.6 million, or $0.42 per diluted share, in the fourth quarter 2017.

The following is an unaudited summary reconciliation of GAAP net income to non-GAAP adjusted net income and per share data, net of tax (in thousands, except per share data):