An investigation by Russian opposition activists has concluded that 220 Russian soldiers died in two major battles in eastern Ukraine.
The report includes data compiled by the opposition politician Boris Nemtsov, who was shot dead in February.
Russia denies Western accusations that it has sent regular troops and armour to help the rebels in eastern Ukraine.
The cost of Russia's military involvement and of annexing Crimea is said to run into billions of dollars.
The 64-page report, called Putin - War, has been published on the Open Russia news website.
At a news conference in Moscow Ilya Yashin, an ally of Boris Nemtsov who finalised the report, said it was the work of "true patriots" who opposed the "isolationist policies of [President] Vladimir Putin".
It details how 150 Russian soldiers were killed in the key battle for Ilovaisk, a small town in the Donetsk region, in August 2014.
More recently, it says, 70 Russian soldiers died in the battle for Debaltseve, which fell to pro-Russian rebels in February, after the Minsk ceasefire deal was signed.
"All key successes of the separatists were secured by the Russian army units," Mr Yashin said.
The question of Russian involvement in Ukraine is highly sensitive in Moscow. The activists said that finding a company prepared to print the document had been difficult.
Supporters of Nemtsov suspect that he was assassinated because of the sensitive information he had collated about Russia's involvement in the Ukraine conflict.
Five ethnic Chechen men are in pre-trial detention, accused of the killing, but prosecutors have not yet established any motive.
President Putin has said the mastermind behind the killing might never be found.
'Compelling case' - Oleg Boldyrev, BBC News, Moscow:
Most of the report is based on facts that have already appeared in Russian and foreign media during a year of conflict in Crimea and eastern Ukraine.
But colleagues of Boris Nemtsov say putting them together makes a very compelling case for accusing Vladimir Putin of waging the war in Ukraine for his own ends.
Mr Nemtsov started this work in early 2015, after hearing of Russian soldiers' relatives who had still not got the promised compensation. But the relatives never went public and - after Nemtsov's death - became even more scared.
Ilya Yashin said the opposition would collect donations to extend the initial print run of only 2,000 copies. But publishing and distributing this kind of dossier is going to be hard. And, above all, the question remains - how many people are keen to learn the facts?
There are plenty of Russians aware of their country's involvement in eastern Ukraine, who nevertheless find it acceptable because they believe Moscow is supporting those who wish to be independent of the government in Kiev.
Mr Yashin and other opposition activists don't call that "support" - they accuse Mr Putin of masterminding and directing the war to boost his failing popularity.
Kept under wraps
A leading economist who contributed to the report, Sergei Aleksashenko, estimated Russia's spending on the rebellion in eastern Ukraine to be 53bn roubles (£665m; $1bn).
The report says relatives of the 150 soldiers killed at Ilovaisk received 2m roubles each (£25,100; $39,000) after agreeing not to reveal how the men had died.
However, relatives of the 70 who died in Debaltseve were given no compensation from the Russian defence ministry, the report says.
By that time soldiers sent to fight in eastern Ukraine were being released from the regular army, to make it look as if they were volunteers, according to the report.
Mr Aleksashenko broke down Russian spending on the conflict as:
- 21bn roubles on the upkeep of 6,000 "volunteer" soldiers
- 25bn roubles on the upkeep of local militia in the rebel-held areas
- 7bn roubles on servicing of Russian military hardware
The others who worked on the report, besides Boris Nemtsov, were: Mr Yashin's RPR-Parnas party colleagues Leonid Martynyuk and Olga Shorina, former Russian Deputy Prime Minister Alfred Kokh, and journalists Ayder Muzhdabayev and Oleg Kashin.