Most people with episodic tension-type headache (TTH) treat themselves with over-the-counter analgesics. In the absence of clear evidence of dose-related efficacy of the two most commonly used analgesics, aspirin (acetylsalicylic acid) and paracetamol (acetaminophen), this study compared two doses of each with placebo. In a double-blind, double-dummy, randomized parallel-groups comparative trial, 638 consenting subjects aged 16–65 years with episodic TTH (but not migraine) by IHS criteria were recruited from the UK general population by advertisement. They treated one episode of moderate or severe TTH with a single dose of 500 or 1000 mg aspirin, 500 or 1000 mg paracetamol or placebo. The primary objective was to compare aspirin 1000 mg with placebo, and the primary end-point was subjective pain relief (total or worthwhile) 2 h after treatment (‘response’). Additionally, pain intensity on a 100-mm visual analogue scale and functional impairment were monitored regularly for 4 h and at 24 h, although rescue medication was allowed after 2 h. The analysis was of the intention-to-treat population of 542 who took treatment (all providing outcome data). Treatment groups were matched at baseline. Aspirin 1000 mg (75.7% response rate; P = 0.0009) and to a lesser extent aspirin 500 mg (70.3%; P = 0.011) and paracetamol 1000 mg (71.2%; P = 0.007), but not paracetamol 500 mg (63.8%; P = 0.104), were statistically more effective than placebo despite a high placebo-response rate (54.5%). Outcome was not affected by headache intensity at baseline. Secondary end-points including functional recovery (by median times of 4.0–13.5 h) were consistent with these findings, although a minority of subjects recorded long-duration functional impairment (37–54 h). Adverse events reported by 13.4–18.9% of subjects were mild or moderate, and transient. No safety concerns arose.