A B737 captain reported returning to the departure airport after losing the a hydraulic system.

Attributes
ACN	1339649
Time
Date	201603
Local Time Of Day	1201-1800
Place
Locale Reference	ZZZ.Airport
State Reference	US
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	B737 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	Climb
Flight Plan	IFR
Component
Aircraft Component	Hydraulic Main System
Person 1
Function	Captain Pilot Flying
Experience	Flight Crew Last 90 Days 170 Flight Crew Type 10000
Events
Anomaly	Aircraft Equipment Problem Critical Deviation - Altitude Excursion From Assigned Altitude

Narrative:

The flight was on time and there were no issues prior to climbout. Upon gear retraction; we saw both a system low pressure lights; initially for the engine pump followed by the electric pump light. We looked for other indications and found the right main landing gear was still in the down position and the a system hydraulic quantity was at zero. We began handling the situation per the checklist and got vectors to a holding pattern to burn off gas. The issue was with the handling of the aircraft. With half the hydraulics and no autopilot; I had difficulty staying exactly at the assigned altitude while trying to deal with the problem. I never got more than 300 feet off the altitude; but approach control was concerned and made several calls reference our altitude. At that point the first officer asked for a block altitude and we were then free to deal with the problem. We eventually returned to [the departure airport] for a normal landing.

Original NASA ASRS Text

Title: A B737 Captain reported returning to the departure airport after losing the A hydraulic system.

Narrative: The flight was on time and there were no issues prior to climbout. Upon gear retraction; we saw both A system low pressure lights; initially for the engine pump followed by the electric pump light. We looked for other indications and found the right main landing gear was still in the down position and the A system hydraulic quantity was at zero. We began handling the situation per the checklist and got vectors to a holding pattern to burn off gas. The issue was with the handling of the aircraft. With half the hydraulics and no autopilot; I had difficulty staying exactly at the assigned altitude while trying to deal with the problem. I never got more than 300 feet off the altitude; but Approach Control was concerned and made several calls reference our altitude. At that point the First Officer asked for a block altitude and we were then free to deal with the problem. We eventually returned to [the departure airport] for a normal landing.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.